Uunderside of the compressor housing, it's run down the tank and onto the floor, from a quick glance I couldn't tell any more.
I guess I'll need to get the housing off to have a sensible look.
Closer look, it's leaked from the annular vent grill.
I went to start stripping, but it was chucking it down, and to get to the compressor enough to really work on it I'd have needed to put a few bits outside, in the rain. I wasn't keen to have wet stuff in there, so I left it for the time being. Hopefully find time to get back in there this weekend.