Check the valley for fuel, not with a lighter!!! lol, The O ring on the fuel/water seperator could be a source, they started leaking shortly after the ULSD arrived. Have you changed your FF lately, sometimes the lid doesn't seal properly and will just let out a mist like stream of fuel under pressure so you may not see a gusher. A mirror on a stick and a flashlight is your friend when looking for something like this.
Good Luck