I have noticed, during some climatic conditions, that there can be some discharge of fuel from the airbox.
On several occasions, if the weather is heating up, the expansion of cool fuel in the carbs has caused a small amount of fuel to enter the airbox and subsequently drain.
On other occasions, when the bike has been run at high speed for long distances, the heat from the engine after stopping causes the same effect.
I should say that the amount of fuel which has vented would be in the vicinity of 20-50ml...
Maybe this is your situation?