Invisible Rain

The building I work in has a metal roof, so when it rains, we always hear it. When I hear rain falling, I usually look out the window. The other day it started raining while I was at my desk. I looked towards the window, but I couldn't see any rain. I went over to the window and looked out, and I could not see any rain falling. I could hear it, but I could not see it ... and I really tried! The rain-dotted parking lot (and the roof) told me it was raining, but it was not visible to the naked eye.