After getting my car back from the mechanic I now have a "low oil" warning.

I checked the oil level with the car warm, but not running, and the oil level looked too high. So then I checked it while it was running and then things looked just above the low oil level. I added a bit of oil and now the oil level ( while running ) is right where it should be. However the message still keeps popping up on my console. Is it possible a simple sensor wire has been knocked off by the mechanic? Any ideas?

By the way this is on my 99 DIESEL E300. Thanks - Dylan