Shouldn't really happen because it only alerts for Airtags that are not with their owner. Then again, they only just released the ability to share an Airtag among multiple iPhone users, so false positives in public transport could be shared Airtags that are with people who haven't set up that feature yet.
So I still believe the case you are describing is Airtags not with their owners. Maybe kids with Airtags connected to their parents' phones. And like I said, the new feature where multiple people can connect to one Airtag might alleviate (not entirely fix obviously) that issue.
Then my colleague probably has the beta because he could activate the feature but not add anybody to his Airtag which makes sense when they're still on stable.