That's about normal and expectable. The primary purpose of the slobber tube, road draft tube, etc is to vent the crankcase of combustion byproducts that arrive there in the form of "blowby", or normal leakage past the excellent but not quite perfect seal at the piston rings. Of the many nasty earth destroying, baby killing, disease spreading toxins that make up exhaust fumes, one of the key players there is Water, and it leaves the engine in the form of vapor. Your can will be very cool (relatively speaking, even though it probably gets warm to the touch), and it will condense a portion of this water vapor. No worries for having found that, it's exactly what you wanted to find. It means the coffee can contraption is working as intended.