I think it was a blocked drain tube. Since I can't leave well enough alone, I ripped apart the passenger footwell last night to see what I could find out. Word of warning: The metal cover the ECU has some very, very sharp edges. The tube runs behind the wire harness for the ECU, and is slightly constricted by the tie-downs. I loosened the tie downs, and as soon as I repositioned the tube, a large puddle was forming under the car. So, either I dislodged whatever was in the tube, or it was constricted enough to cause this leak. Car will still be going to the dealership tomorrow to get checked out.