My S-class behaved exactly that way; turned out to be the little fan that drew cabin air over the in-cabin temp sensor. I think it's called a "sampling fan" ($50). On my S-class, the cabin temp sensor is behind a little grid in the roof console just above the mirror. A rubber tube routes all the way down to the right lower dash below the glove box, where the fan sits. It just sucks cabin air over the sensor to maintain proper temp readings.
The SL sensor is probably behind a little grid somewhere on the dash.
A quick check is to turn the ignition on and see if a light piece or tissue paper will adhere to the grid (suction), then fall off when the ignition is turned off.

DG