My first guess would be coolant from the expansion tank.
Any cooling issues to note lately? High temps? Any liquid on the ground after you park?
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