The leveling pump is mounted to the block, just under and left of the distributor. I expect it is possible the pump could be leaking internally into the crankcase. If that is the problem, you need to replace or reseal the pump and get the motor oil changed. Hydraulic oil is much, much thinner than motor oil.
You say the oil level came up in the engine, did it come up about 2-3 liters? If the level came up, it is very likely the pump is leaking into the crankcase.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.