How about the simple fact that when dispatched, the aircraft has much more fuel in the tanks than when it arrives.
It's been a while, but I think I recall that the inboard upper wing surface on the MD-80 series was not "exposed" to fuel until the tank had more than a certain amount of fuel in it. The "exposed area" crept outboard as the tank was filled more.
I remember something about "Ice 9" inspections after long high flights, and only the very inner upper wing surface was affected.
So if the leak was on the upper wing surface, the further outboard it was, the less likely to be seen, unless the airline was in the habit of carrying lots of fuel.
Certainly unlikely on arrival.
Of course this re-inforces a policy of not doing final preflight inspection until all fuel is on board.