Racine has a pair of lesser adjutant stork. They probably weren't seen because of the cold weather; the species is native to warmer areas such as south east Asia.
Maybe it was too hot for them those days (I don't know much about the species)? How do you know that they weren't out before or after you saw the exhibit? How do you know they weren't just hiding from the public those days? How do you know they weren't delivering some babies? What makes you so sure the zoo doesn't have them besides the fact you didn't see them the five times you went? Pachy pro says the zoo still has signs for them and the website still lists them. I think it's safe to assume they still have them.