Probably midgets. Possibly gnomes, depending on the climate. They weasel their way in, especially on earlier models. They tend to nest in the brake booster and crawl through the vacuum line to the air intake.
Ahhahahahaha I kid..
I don't think it's a radiator leak or hose leak, because there'd have to be alot of air being drawn into the box, and the air filter would probably soak most of it up anyway, since you said the coolant is on the motor side of the filter.
Good luck.