"Mr. policeman my phone got stolen from my hand". Then got used to open the door for the 20 flock that robbed the store blindly. Prove that he's lying.
There are a few reasons this is an unlikely scenario:
1) You need to unlock the phone to access the Amazon Go app QR code
2) There isn't just a door to the store, there are security portals that make it difficult to tailgate
3) If a thief is bold enough to steal a phone, they're not interested in petty convenience store theft, they're going to flip that phone for cash