Quarantine
Vet
Moved to out-of-sight holding enclosure while their forever-home is getting fixed up.
On loan to another zoo.

Same with those with babies without parents. Baby doesn’t mean “newborn”. Baby could well be more than old enough to fend for itself, particularly in a place where it gets fed and sheltered from predation etc. Perhaps your baby has come from another zoo. Or parents are being given shots. Or are resting out of sight.