I have the same problem! Thought it was the oil drain plug also, dropped the belly pan gave everything under there a good cleaning and watched and watched. THought it might have been the crush washer on the drain plug when I changed the oil last so I had one in the tool box and replaced that as well. NO Oil! After a few days I check again and the oil pan itself seems ok and the belly pan is still oily or light residue but not nearly like before an the drain plug is clean and dry.

Dropped the plastic tray in the front! Ah, right where the compressor bolts up there is a tab on the oil pan gasket! Seems to have a drop of oil there. My indy seems to think it is the oil pan gasket. If you don't let them at your drain plug with a pneumatic tool at the quickie lube and torque it should not be the plug or the pan. Probably the gasket.

Mine is an 8.02 3.0i