Originally Posted by arejohn

If its anything like a Volvo I had this device can be quite sensitive. When I replaced an out taillight bulb the indicator did not go out. I had to replace both tail light bulbs. Could also be a corroded socket or connection, ground etc.

I don't know how the MB checks bulbs but Volvo was comparing the current draw of bulbs in pairs- 2 headlights, 2 taillights, and on.