Fake valets? Yes, from time to time it happens. You should see a name tag and uniform. Snap a photo of their face with your smartphone. Ask them questions:
What's the cost?
What's the procedure for getting my car back?
Can I call a number to summon my car before I arrive back at this spot?
What's the name of the valet company?
How long have you been working here?
What's your name?
I'd like to see your driver's license. (Snap a photo of it.)
If the person posing as a valet hasn't run away by now, they're probably legit. But if you get any intuitive feelings that tells you something isn't right, then just get in your car and leave. No need to be polite in this situation. Go! And park elsewhere.
Sometimes the valet isn't really the valet.
It isn't entirely clear how the Mercedes got stolen. It may have been sitting on the "ramp" with the keys in it. Somebody may have stolen the keys from the cashier or valet stand, and then stolen the car, or somebody may have used other methods.
Most likely, it was left on the driveway with the key fob in it, and somebody merely sat in it and drove away.
Above: Interesting insights into somebody's friend.
Don't leave any valuables in the car.