I believe the tracker doesn’t emit the same signal when the phone the AirTag is paired with are communicating directly. So as long as the owner of the airtag is in the vicinity it will not be considered as following/stalking you.

It can tell because the random guy’s Airtag will be connected to his iPhone, it will only notify people if it isn’t close to the owner’s device

I’m pretty sure it’s length of time. If you’re on a train next to someone, you won’t be for more than an hour or two. Not two days.

When the device follows you constantly for multiple days is likely when it’ll make the alert.

It only notifies you if the air tag is without ite owner. If this wasn’t the case I’d be getting notifications left and right for my wife’s airtag. Instead I only get notifications when her phone is left behind but her tag is with me.

When I visit family via train, google knows I was on a train ( maps timeline ), so maybe they can tell by how straight the travel is ?