I own a 2000 Ford Taurus.  Recently I started to notice that when the
car sits for a long period of time (overnight for example), it leaks
enough fluid to form a small puddle behind the left rear wheel.  I
don't smell gas, and when I wipe the stuff off the floor, it does not
appear to be gas, but I don't know what else it could be.

When I crawl under the car and look up, there is what appears to be a
rubber grommet that is parallel to the floor, on the flat surface of
the underbody, a couple inches rearward of the left rear wheel well.
There is evidence of fluid leakage around this grommet and there
appears to be evidence that the substance has landed on just about
everything in the area (including the muffler).  Does anybody have any
idea what this could be?  If it is gas, I would imagine that having it
leak directly on the muffler could end up disastrous.