If someone sticks one to your car, they can track everywhere you go and you’d never realize it. The big thing with AirTags is that they aren’t just local tracking. They have access to the entire FindMy network, which is comprised of every single iOS device. So any time you drive past an iPhone, the AirTag is able to report in with your latest location.
With an iOS device, you’ll get an automatic alert if you’re being followed by an unfamiliar tag that hasn’t connected to its paired device in a while. So like if you’re with a friend, you won’t get alerts for their AirTag because the tag is near your friend’s paired phone. But if that friend drops their AirTag into your bag and you go home, you’ll get an alert that you’re being followed by an AirTag.