Leave some peanut butter or cheese out and see what happens to it.
Put down several layers of newspapers to capture some of it. See if it comes from underneath the paper, or on top.
It's only on horizontal surfaces?
Do water droplets bead up on it or soak in?
See if it's flammable. There are also simple checks for pH.
Your version of the EPA might be interested in this case. There are "electronic noses" but they are mostly built to detect explosives.
Any high school chemistry teacher may be able to devise more tests, perhaps using Reagent - Wikipedia, the free encyclopedia
Do you live near a university?
Chromatography - Wikipedia, the free encyclopedia
At least you'll know what you're up against.