On my 86 560sl it took me quite a while to find this one. My only hint was when I removed the oil fill cap the gas odor got worse.

How to check the fuel pressure regulator vent tube. It is a hard plastic line that runs into the rubber hose that connects at the rear of the passenger side valve cover. If the diaphram goes bad it will slowly dump gas into the engine so you will never see a leak. With the fuel lines under pressure if you pull out the hard line out of the rubber hose and it drips gas you will need to replace the regulator and oil ASAP. You may need to check it at the regulator itself if the hard line is disconnected at that end.

I was lucky and only got a good internal engine cleaning. Others have not been so lucky and have ruined bearings because the oil got to thin.