If you're talking about the pink/blue paper collars, those are on for identifying animals name, number, and gender. If they don't have one of those on, the collar either came off and no one's replaced it yet, or the cat may have just gotten there recently. If you mean a real collar, some cats are owner surrendered or brought in as "strays" with them on, or the staff may have put one on for event purposes.
Some were probably given to the shelter from a litter from a private party... I'm guessing???
You are both correct
Thanks! Your answer is awaiting moderation.