Agree with the catch can. Did you happen to just do an oil change or just add more oil? If you are a hair over what the Bullet can hold, it'll regulate itself from that catch can in that way you described.
I don't have an air filter box on my AVL Electra. Just the catch can mounted behind the battery cover.
The three hoses Dan mentioned come from the engine, into the can. One hose exits the can and dumps the "mayo" out... hopefully not in the path of your rear tire.
Word of caution, if you take this apart to clean, make sure none of your hoses are crimped or pinched when you put things back together. I pinched my exit hose under my seat and spent the next 2 weeks of the engine running terrible and me fiddling with my carb before I noticed the hose.
Boggy