The brief
The A180 had oil pooling beneath the oil level sensor, the level dropping between checks, a dashboard warning light, and the oil level readings going inaccurate. He brought it in before the level got away from him. On these the oil level sensor sits low in the sump, reads how much oil is in there, and seals to the pan with an O-ring. Over years of heat that O-ring hardens and the sensor itself can fail, so oil weeps past it, which is the pooling and the dropping level, and the sensor stops reading correctly, which is the inaccurate readings and the warning. A slow oil loss is easy to keep topping up, but an engine that runs low on oil is one you don't want to drive on, so it needs the sensor and its seal changed.
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. 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 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, the level held, and the readings were accurate again.
The outcome
Dry sump area, no pooling under the car, the oil level holding, accurate readings, and no warning light. The A180 went home with the leak closed off. An oil level sensor leak is easy to keep topping up and forget about, right up until the level gets away from you, so sorting it kept it to a tidy job.