It's theoretically possible (although I'm generally not a paranoid person) that someone could've attached some kind of tracking device on your vehicle. Again, not a paranoid person, but the possibility is not zero, mathematically speaking.

Edit: just an idea: google maps also will let you share your location indefinitely, but it will notify you every so often that you're sharing location. If this has started happening recently, and if you have google maps, check in there?