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So this guy unlocks your car and comes to get you, I presume leaving the car unlocked?
In which case an opportunist thief could have taken your sat nav, it seems unlikely to me that the guy helping you would have taken it, I could be wrong but lets face it he is there everyday easy to trace and knows it.
So why not go back to the store and ask if they have surveillance in the car park that covered where your car was? It may show who the culprit is.