Same problem on my S500/W220/2000. It just came on again. Haven't had it in to the shop yet, but have removed and replaced the fuel cap a few times and the light eventually goes off, but it has started coming back on regularly. Called the service guy and said to bring it in if it stays on for a few days. Please report back here if you learn anything useful. I will do the same. This seems to be something many owners report on a regular basis.