Yeah, according to Apple it’s all about stuff that you’ve lost. Once separated from you after three days that’s when it will start chirping. It will only alert someone sooner than that if it’s continuously with them and they have an IPhone (if I understand correctly this is the only way it knows that it seems to be following someone). The point behind this is that they don’t want someone using an Airtag to stalk/track someone else.
It’s not meant as a theft deterrent. Additionally It can be disabled really easily, considering it has a user replaceable battery.