Antwerpen Zoo (Belgium). The Brahminy starling (Sturnus pagodarum) was signed in an outdoor aviary complex near red-cowled cardinals, but didn't found them.
Apenheul Apeldoorn (Germany): it was a rainy day and the Silvery marmoset (Callithrix argentata) group was hidden. Crowned lemur (Eulemur coronatus) and Alaotra gentle lemur (Hapalemur alaotrensis) were also no-shows. I arrived to the place just when the Mississippi paddlefish (Polyodon spathula) deceased? or moved? from the sturgeon pond.
Aquarium Morsky Svet (Czech republic) - they kept Black arowana (Osteoglossum ferreirai) in the big mixed central freshwater tank, but mixed with common silver arowana and I was unable to see one that looked like different.
Artis Royal Zoo - Amsterdam (Netherlands) - at Micropia they supposedly keep tardigrades during the time of my visit, but I didn't found an enclosure for them.
Barcelona Aquarium (Spain) I've seen the signage for the wonderful Angular rough shark (Oxynotus centrina) in the oceanic tunnel, but absolutely no sight of the animal.
Berlin Tierpark (Germany). When I arrived the seabird outdoor aviariy was under renovation so I missed things such as the Grey gull (Larus modestus).
Berlin Zoo-Aquarium (Germany). The first thing we visited here was the Nocturnal House for arrive before lights turn off. No luck on this side. In the nocturnal house there is a enclosure for Common vampire bat (Desmodus rotundus), but only saw the signage. Also no luck with Tuatara (Sphenodon punctatus) in the reptile house, a very big enclosure, but no reptile seen inside. I also checked every corner of the Coconut crab (Birgus latro) enclosure, but absolutely nothing of it was seen.
Best Zoo (Netherlands) - not sure if this counts, but I visited it with Vogelcommando and he asked if we could see the Derby's woolly opossum (Caluromys derbianus) that is kept BTS and that many zoochatters were able to visit and even hold and cuddle just by asking. But we had no luck.
Birch aquarium (USA): Clipperton's grouper (Epinephelus clippertonianus) signed in the sunken ship tank, but apparently missing. Pacific spiny lumpsucker (Eumicrotremus orbis), shares a small tank with grunt sculpin, I checked every corner of it but no sign of lumpsucker.
Bronx Zoo (USA): an extremely heavy repentine rain happened in evening. After that rain, all the animals in the outdoors were hidden. Including the Nubian ibex (Capra nubiana) that share enclosure with geladas (also not seen here). Also I lost a lot of valuable time waiting for the rain to go, and I was unable to visit the Butterfly Garden (already closed when I arrived), the Congo Gorilla Forest, the pheasantery and the Wild Asia monorail. The Grey-necked woord rail (Aramides cajanea) was not seen in the enclosure for it.
Burger's Zoo (Netherlands) Ringed seal (Pusa hispida) went out of the zoo for the construction of a meerkat exhibit, just a month or so before my planned visit :-( Other species that I wanted to see, and signaged, present during my visit but not seen were Rose-bellied bunting (Passerina rositae) (Desert & Bush), Chaco chachalaca (Ortalis canicollis) (Bush), Guadeloupean anole (Anolis marmoratus) (Mangrove?), Moon-tailed bullseye (Priacanthus hamrur) (Ocean), Top minnow (Belonesox belizanus) (Mangrove), Pinon's imperial pigeon (Ducula pinon) (Bush) and Honeycomb stingray (Himantura uarnak) (Ocean).
Duisburg Zoo (Germany): Hottentot teal (Anas hottentota) signed in the pelican pool, but not seen. Regal girdled lizard (Cordylus regius), famous no-show, rarely out of its hole. Brown ghost knifefish (Apteronotus leptorhynchus), shares tank with much more common A. albifrons, but no leptorhynchus seen, Lantern bass (Serranus balwini) rarely show itself. Monkey goby (Neogobius fluviatilis) (present theorically in the giant salamander tank, also visited a small aquarium for it behind the scenes but it disappeared mysteriously). Emerald catfish (Corydoras splendens), signed but not seen.
Fuengirola Zoo (Spain): Hottentot teal (Anas hottentota) signed, not seen.
Kölner Zoo (Cologne Zoo) (Germany): Red-footed falcon (Falco vespertinus), signed, no-show in an aviary shared by other birds. I was fortunate to visit behind-the scenes with staff, but just after the dreamed Flame-breasted fruit dove (Ptilinopus marchei), the only one in European zoos, left the zoo for join a pair in private hands for breeding. White-browed coucal (Centropus superciliosus), signed but not seen. Vietnamese forest lizard (Calotes bachae) shares a terrarium with Gekko badenii, G. grossmani and Polypedates megacephalus, but I only saw the sign. Yellow-headed gecko (Gonatodes albogularis), one of my most wished of the place, but the terrarium looks like empty. Vietnam crocodile newt (Tylototriton vietnamensis), I remember the tank for it, isolated from the others, but nothing seen inside. Strawberry poison frog (Dendrobates pumilio) theorically inside a terrarium that is inside the butterfly house, but no strawberries seen. The Silver tiger fish (Datnioides polota) lives in a pond in the jungle greenhouse, I've photographed Datnioides here, but they resulted to be the much more usual D. microlepis instead.
Madrid zoo-aquarium (Spain): Lesser kestrel (Falco naumanni) signed on the map, in/near the children/farm area, I was unable to locate the enclosure despite following the map.
Ménagerie-Jardin des Plantes du Paris (France): Riobamba marsupial frog (Gastrotheca riobambae), terrarium signed, but the frog didn't showed.
Mořský svět (Czech republic). Atlantic giant isopod (Bathynomus giganteus) signed for a tank where I've only seen chambered nautilus and Japanese pineapplefishes.
Pairi Daiza (Belgium): The most punching one was the mythical Bear cuscus (Ailurops ursinus) present in the Oasis. I've passed by its enclosure twice, but abdolutely no signal of it. Only the signage. And theorically it's an easy to see individual... But I had no luck. Other animals present here and in public, but not seen, includes Brown-hooded kingfisher (Halcyon albiventris), signaged in the walkthrough aviary. Lake Urmia newt (Neurergus crocatus) lives inside the bat cave, that I visited during closing hour and with no time for exhaustive search I didn't found the aquarium for it.
Plzen zoo (Czech republic). I've visited the bird room behind the scenes, but I didn't knew that there is also a small mammal room behind the scenes nearby! By the time of my visit I was obsessed with birds so the zookeeper friend that was with me carried to the bird room only.
Prague zoo (Czech republic). In the Africa at Night houtse, there is a large pannel for viewing subterranean tunnels of Mechows' mole rat (Fukomys mechowii), but I didn't saw the rodents.
San Diego Zoo (USA): the most punching of all! Went here with the very main objective of California condor (Gymnogyps californianus), but when I arrived, their enclosure was under renovation and the condors behind scenes! Very punching also was the missing of Crowned eagle (Stephanoaeus coronatus), I had little time for see the whole zoo and somehow I missed the patch (part of the Hippo Trail) where the aviary for it is. Also the Coconut crab (Birgus latro) was a great missing, I was unable to find the enclosure for it.. In the Australian Outback I remember signage for Squatter pigeon (Petrophassa scripta) but the birds was no-show. When I arrived to the Allen's swamp monkey enclosure they should be Spotted-necked otter (Lutra maculicollis) playing with the monkeys, but absolutely not even the tip of a tail of the otters appeared despite exhaustive seeking. Also by lack of time for seeing all I skipped the presentation where they show the Tree pangolin (Manis tricuspis). And in the Hummingbird Aviary I was unable to see many of the small tropical tanagers and other birds signaged such as Golden-collared manakin (Manacus vitellinus) and Swallow tanager (Tersina viridis)
SeaWorld San Diego (USA): missed the rare Guadalupe fur seal (Arctocephalus townsendi) as well as the Ringed seal (Pusa hispida), I didn't saw signages for them and in the sea lion pool I only saw California sea lions...
Taipei Zoo (Taiwan): Insect House closed for renovation just during my visit. I wonder how many wonderful rarities they have... In the big walkthrough aviary are signage for Whistling green pigeon (Treron formosae) (found feathers of it in the walkthrough aviary, but not the birds), Ashy wood pigeon (Columba pulchricollis) and Malayan night heron (Gorsachius melanolophus).
Wuppertal Zoo (Germany): in the snow leopard area found a wire cage full of rock that houses Turkmen mouse-like hamster (Calomyscus mystax). Needless to say that a nocturnal rodent in an outdoor big enclosure full of rocks as hiding places, are absolutely unseen. Ansell's mole rat (Fukomys anselli) lives in the indoor part of the small cat house, full of dead birches as decoration. I've been here but failed to see the terrarium for it. I had the bad luck that the walkthrough aviary at the end of the bird house was just closed (for renovation?) during my visit, inside lives Lesser kiskadee (Pitangus lictor) and pacific Pale-legged hornero (Furnarius leucopus).
Zie-Zoo (Netherlands). I arrived late and it's a bit rainy. The absolute main objective of the zoo, the Plains viscacha (Lagostomus maximus) was absolutely off show despite passing many times by its enclosure. It shares the same enclosure with Yucatan squirrel (Sciurus yucatanensis) equally hidden. Another squirrel species, American red squirrel (Tamiasciurus hudsonicus) lives in a part of the zoo that I had no time to visit (near closure).
There must be many other exaples that I already whiped out from my memory. But at the moment these are the highllights of "I almost got them" and all are species that I still never saw in captivity (nor in the wild).