I have a 2004 Vito that does the same and on occassion , it also locks the key in the ignition switch and can't be removed on other occasions, I happens with both keys I own and ther'e no pattern to it.
I've also been unable to get into the vehicle, the worst case was 20 minutes, it just wouldn't unlock, fortunately I was close enough to home to go and get the spare key, which then didn't work either, then all of a sudden the riginal key did work - no rhyme or reason.
I've come across two other Mercedes stuck in Petrol Stations (both saloons) that have had a similar problem.
My dealer flatly denies there's a problem, and of course you can never get the problem to re occur whilst at the dealership, I also happen to have a host of other minor elecrical gremlins, some of which have never worked since the day the vehicle was delivered, yet it appears Mercedes are unable to repair them.
Which is why I'll not be replacing this vehicle with another mercedes when the lease is up...
I also own a 1990 560 sec which has a metal key that you put in a lock and turn it....has no electrical gremlins and aprt from being a little tatty ( repsray organised) is a fine car. Shame about the overcomplicated stuff.
Alan M