Hi, My name is Paul & will try to assist.
This is not too unusual to be honest - owner pops the boot using the key fob, puts the ket in the boot, takes out shopping & shuts the boot again. Vehicle remains locked!
There are few options Im afraid
1 - best option is to gain entry using your spare key - if no key is available then a dealer can usually emergency order a key for the next working day.
2 - if the key is accessible then break one of the rear lamps - force past the interior trims & reach in to get the keys
3 - I have heard of some locksmiths being able to gain entry using by picking the mechanical lock in the boot lid. The laser cut keys on a Mercedes are not easy to crack & is not something I have ever arranged, but have heard of it.
the spare key is certainly the best & easiest option
Best regards, Paul