A Faulty bulb can cause this.
I had a faulty bulb in my 3rd brake light, and it would cause the indicator to come on from time to time. the bulb was still lighting though, but intermittently it would not.
I replaced the bulb, in fact I replaced all the bulbs besides the interior bulbs on the car at that point, didn't cost much, and I know they are all new.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.