From the accounts here, it sounds like it's pretty easy to activate Summon when you just want to park your car, by accidentally pressing "park" twice instead of once. This is compounded by the fact that there is no "off" for Teslas, you're expected to park it and get out. Sure, there might be dings and dashboard lights, but I can easily see how someone might not see them, or might not know what they signify is about to happen. Finally, as far as I'm aware, there is no external sound or indication that the car is in Summon mode, it just starts to move, so I can easily see how, once you exit the vehicle, you'd be unaware of it happening behind you.