New Moto Guzzi Door Mats Available Now
It's not actually a "leak" - the float bowl vent is nearby and gas fumes vent up and dry around the hole. Totally normal for Dellorto VHBs and nothing really to worry about. If it bothers you, spray some carb cleaner, Purple Power, Super Clean, etc. on an old toothbrush and scrub it away.
A good tap on the bowel usually fixes it temporarly.