Could be coming from the tank itself. Maybe a seam or a small crack in the tank at a weld seam. I have seen this before and the fuel finds a path to follow so it may not be obvious where it is coming from. The fuel from the main tanks is routed from the tanks to the boom where there is a small sump then to the fuel selectors. The aux tanks are plumbed directly to the selector and bypass the sump in the boom. Which tank is leaking? Mains or Aux?
Mud daubers and bugs can clog the vents. This should be cleaned but can be a bugger (no pun
. Check the forum for other posts on this issue using the search feature. Also there is a procedure in the service manual on testing and checking the fuel vent system. There are some check valves involved.
You may have to pull the Structural panels on the upper wings to perform the repairs. If you do remove the panels be sure to follow the service manual guidelines to support the boom and the wing. Those panels are Structural to the wing.