Battery drain on '90 300CE

I am hoping that someone has an idea on the best way to handle the problem of something drawing on the battery of my car overnight. I do not find anything left on such as a light or anything else. There is a motor that stays on for about 60 seconds after I lock the doors. The motor seems to be located on the passenger side of the car's bottom near the front seat. I wonder what motor that might be, and how I might silence it after I lock the door?

But I suspect that the reason that the battery has been drained completely at night is something else. I stopped by an electrical shop earlier, and the owner told me that the problem normally with Mercedes autos is human error, rather than a short, but suggests that I leave the car to be checked out. Before I do that, does anybody have any other ideas? Thank you very much in advance for any help.

Mike Hartigan