The brief
The CLA180 was dropping engine oil, a film building up under the engine and the level creeping down. He brought it in, which is the right call, an oil leak only spreads and an engine low on oil is an engine at risk. The trail pointed at the oil level sensor. The sensor sits low in the sump, screwed in through the pan, and reports the oil level to the car. It has a seal where it passes through the sump, and that seal heat-cycles for years until it hardens and weeps, so oil seeps out around the sensor, runs down the pan, and drips. The sensor itself can go faulty too. A weeping sensor seal doesn't reseal itself, so the sensor comes out and goes back in with a fresh seal, the sensor renewed at the same time.
The diagnosis
With the engine cleaned off and run, the leak traced to the oil level sensor in the sump, oil weeping from around its seal, not from the sump gasket or anywhere else. The rest of the engine's seals checked out dry, so it was the sensor seal. That's a sensor-and-seal replacement: a new genuine oil level sensor with a fresh seal, fitted properly so it seats and seals.
The work
The undertray came off and the oil was drained enough to get at the sensor, the old oil level sensor removed, the seating cleaned up, and a new genuine Mercedes-spec sensor fitted with a fresh seal, torqued to spec. Fresh oil to level, and the engine run and checked warm for any weep. A road test confirmed the area was dry, the level held, and the oil level reading was steady and correct.
The outcome
No more oil under the engine, the oil level sensor sealed and dry, the level reading correctly, and the level holding between checks. The CLA180 went home with the leak resolved. A weeping sensor seal only worsens and quietly drinks oil, so renewing the sensor and the seal stopped the leak at its source.