[davew]

I randomly get a warning to "top up engine oil" even though this has been done. The sensor was changed but approximately 6 months later I get the same message. My Citroen garage topped up the oil once again but a few months later the message pops up again. There is no visible leak which I would have noticed. Does anyone have any clues what the problem might be?

What engine do you have? And how many miles between the top-up and the low oil warning?
The petrol 82 engine apparently has a small oil capacity so possibly the normal engine oil usage is sufficient in the few months for the level to drop.

The engine oil dip stick will give you a visual indication of actual level. The User handbook explains how to do this check, and suggests checking and topping up, if necessary, every 3000 miles.