Historically, the glues and adhesives that are used to attach dash duct work together are often times temperature sensitive and tend to emit strange odors, especially when there is a temperature chagne associated with a humidity change. I doubt it is liquid from the air intake vents or the area below the wipers, but it is the glue that holds the duct work together "reacting" to significant temperature changes.

It has been said:
Hybrid drivers come in 3 flavors, greenie, techie and cheapie. Pick any 2.
2005 Prius, Melinium Silver over gray, package 5 (AI)