I tried this yesterday with a tag that wasn't tied to my phone. I heard or saw nothing until I got home, where a notification popped up innocently telling me a tag had been traveling with me. When I went into find my to see it, I could see a (fairly accurate) map of my 2 hour travels and the opportunity to disable it. I would have never noticed it if I hadn't looked at my phone, and it took 2 hours and arriving home to be notified. So it's useful as a temporary anti-theft device, but not for tracking someone without their knowledge (good!). Unless they're an android user, or have an older version of iOS - then it's 3 days.