I had been running on cross feed for a bit during run up, partially to not have the fuel in the cross feed lines be ancient stagnant fuel as you correctly pointed out the only real time you would do it is for extended single engine operations (i.e., already up shit creek). The leak occurs when the selector is in anything other than the normal position, the leaking fuel came around the inspection plate that gives access to the selector valve in the wing (just outboard of the gear).
