The last time I saw that on my 300E, it was time for a new coolant pump.
Ironically, the coolant weeps out of the coolant pump and drips onto the block. It is very difficult to trace, and yes, the pump looks bone dry.
I had to wipe off the area and just stand there for several minutes and wait for the trickle of coolant to show up. Sure enough, eventually, it did, and when I placed my hand near the source, I caught drips coming from the pump (where you can't see it of course).
Mine didn't just collect on the belly pan...it had begun to puddle on the garage floor...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car