940GLE '91 Leaks on rear driver side

[bobbyz]

Washed the car and parked it away. Later in the day I found that there's leakage at the rear driver (left) side, just after the gas tank shield as shown in the pic.
Seems the leakage's been there for some time but I didn't notice it.
It looks pretty fresh too on paper towel, so I guess it's still happening(?)

It doesn't smell like motor oil, gear oil, tranny fluid, power steering fluid or brake fluid. Definitely not gas.

Any idea what is it?