I wouldn't expect an MB dealer to be any better at tracing this that your mechanic. If the current draw is only 60ma, then this is less than 6 amp hours over 4 days. As you already said, this is not enough to drain your battery completely.
I would suspect other things. It is possible that your battery is faulty. Even if it appears to accept a full charge, and will start the car OK, it may have an internal fault that causes it to lack its full capacity. Try disconnecting your battery and letting it stand for four days. Reconnect it and see if it has much power left.
I think I'd be trying a brand new, high quality battery before I spent hard earned cash at the dealer.