I've heard of that sort of thing happening with physical keys. In fact, it happened to me once -- I accidentally opened another Saturn's trunk when mine was parked a few spaces down. This is the first I've heard of it happening with Teslas and their phone-as-key functionality, though, and I find it surprising that it would happen at all; as I understand it, the "keying" is based on Bluetooth addresses, which are (or at least, should be) unique, and there's no such thing as "close enough" when it comes to a matching address. That said, there have been successful demonstrations of "hacks" of Teslas' phone-as-key functionality (see
here, for instance; but there have been other reports). There's a chance that the other Tesla owner was playing with such a technique, you caught him in the act, and he dissembled about it -- but then again, Hanlon's Razor may apply here ("never attribute to malice that which is adequately explained by stupidity" -- or in this case, a software bug). In any event, I do have some suggestions for "hardening" your vehicle:
- Apply PIN-to-drive -- This feature requires you to enter a number before you can drive the car. See the Tesla owner's manual for details.
- Remove unnecessary keys -- Every key (key card, key fob, or phone) that the car recognizes is a potential threat, either because somebody might steal a key or because of overlap (an accidental duplication of Bluetooth addresses, for instance).
- Use an RFID-blocking wallet -- If you keep your key card on your person, store it in an RFID-blocking wallet. This will reduce the risk of an RFID-cloning attack on your car.
- Lock your rear seats -- I haven't heard much about this recently, but soon after the Model 3 was introduced, there were news stories about thieves breaking the small pane of the rear window to unfold the rear seat to see if there was anything valuable in the trunk. If the thief spotted valuables, a bigger window would then be broken so as to gain full entry and steal the trunk's contents. In response, various third parties started selling locks for the rear seats, typically accessible only from within the trunk, along with stickers to put on the rear windows to warn thieves that this trick will be useless. This obviously won't help with the sort of electronic access problem you've reported, but I thought it worth adding to the list.