The leak could well be the fwd sump drain. On the cfm56-3 the fwd sump has an overboard drain which will normaly never be used,it sits at 6 oclock on the fan frame and looks like a boss with a hole, ie no drain line to it.The idea behind it, is in a normal eng, air across the labirenth seals of the fwd sump is of a higher value than the oil pressure within the sump. So air goes in and the oil stays in. But if the lab' seals are worn or tear then the balance can be lost. This is rare and usualy shows with a decent,the oil floods the seal due to a/c attitude,air pressure being low with idle set and the scavange system running slower. Oil leaves the sump into the sump shroud and o/board via the drain. Symtems oil loss, wet cowls but no leak on the grd with a level attitude.fault isolation is to bag the drain with a plastic bottle that has a hole at the top so to allow venting(air is always comming out of this drain so will blow up and pop a bag) fly and check.No luck,bag all drains fly and check.No luck engineers chalking powder and dust the casings, check. Good luck.