AirTag is designed to discourage unwanted tracking. If someone else’s AirTag finds its way into your stuff, the network will notice it’s treling with you and send your iPhone an alert. If you still henʼt found it after awhile, the AirTag will start playing a sound letting you know it’s there.
Of course, if you happen to be with a friend who has an AirTag, or on a train with a whole bunch of people with AirTag, don’t worry. These alerts only get triggered when an AirTag is separated from its owner.