Get under the car or use a mirror to look at the head gasket in the rear exhaust manifold area. My 97 SL320 ( with a M104 engine same as yours ) is starting to weep coolant there as well above the water pump.

Also have a look at the AC condensate drains, if a heater core is leaking, most cars ( regardless of brand ) will drip coolant there. Some older stuff will flood the interior though.