Jay can answer better but he lives fairly close to the zoo and visited fairly regularly so it's not really likely that one would have been there, especially signed, without him having seen it. A releasable rescue wouldn't have gone to the zoo, either, it would have gone to a rehabilitator (or to a collection with a rehabilitation center for marine mammals, which Smithsonian does not have) where it would have had as little Human interaction as possible before being released. Spotted Seals are also native to the far North Pacific, so any rescues would have gone to the Alaska SeaLife Center-- which is where Mystic's came from-- and definitely would not have been sent to Washington, D.C. only to be sent back for release months or potentially weeks later. Is it possible you saw a sign that talked about the spotting pattern on some Harbor Seals? Can you share your photo?
~Thylo