The brief
The B180 had been losing oil slowly, with fresh spots showing under it after it had been parked, and the dash had started flagging low engine oil. He brought it in before the level got away from him. On these the oil level sensor sits low in the sump, where it reads how much oil is in there, and it seals to the pan with an O-ring. Over years of heat that O-ring hardens and the sensor itself can fail, and oil starts weeping past it, a little every time the engine runs, which is the spots on the ground after parking and the level creeping down. A slow loss is easy to keep topping up, but an engine that runs low on oil is an engine you don't want to drive on.
The diagnosis
On the lift the leak traced cleanly to the oil level sensor at the base of the sump, weeping past its O-ring, with the sump pan and the other seals around it dry. The sensor was also reading erratically, so it was failing as a sensor too, not just leaking. That's a replacement. The sensor and its seal are a sealed combination, you don't patch a hardened O-ring on a tired sensor, so the unit gets changed.
The work
Enough oil was drained to drop the sensor clear, the old oil level sensor removed, and a new genuine Mercedes-spec sensor fitted with a fresh O-ring, the connector clicked back home. The oil that had tracked down was cleaned off, the engine topped back up to the correct level with the right spec oil, and held at idle to confirm the new seal was dry. A road test followed to confirm it stayed dry and the level held.
The outcome
Dry sump area, no spots under the car, the oil level holding, and no warning lights. The B180 went home with the leak closed off and a clean underside. An oil level sensor leak is the kind of thing that's easy to keep topping up and forget about, right up until the level gets away from you, so sorting it on the first spots kept it to a tidy job.