Apple Airtags

take the helmet with you...can be a real pain in the rear but if you are going to have to deal with someone who feels its ok to take something that belongs to someone else, then you are dealing with someone not wrapped too tight and who knows what else they may feel they are entitled to do to keep the helmet,,,lets face it, in todays world, nothing surprises me anymore what people think they are entitled to.

I've never actually seen an airtag in real life, but if it's thin enough to fit in the helmet without you feeling it, go for it. I would probably try to secure it to keep it from sliding around in there. Know that it does carry a bit of a safety risk though, as the helmet is designed to take impact at is. Putting anything in or on the helmet creates a chance that there could be a malfunction when you need it the most. That said, we all ride motorcycles, so risk acceptance is what we do.

I can understand wanting to put a tracking device in your helmet. Whiskey brought up a good safety point.
Several years back a coworker of mine went into a store for a minute and came out to find his helmet stolen, he put it over a mirror. He was in the store long enough to grab a bottle of water and pay for it. He ended up having to call his dad and asking him to go to his place, grab his other helmet and bring it to him so he could ride home.
A tracker will help you locate your helmet after the fact but you will still have the inconvenience of having to get another helmet to get home if you live somewhere where helmets are mandatory.