You will find these plugs on all Mercedes engines. If they are not used in some market for a water passage they are just plugged and plugged good. It is very rare for one of them to leak.

Some of these ports are just to clean out the sand that was left from the casting process. But they are then sealed up forever as they are not meant to functions as freeze plugs.

Go to a car wash and clean that part of the engine. Then drive home. The leak will be quite apparent when the engine is clean as the only part covered with fluid is the part where the leak is.