It once took me about that long to find a leak in a 78 Thunderbird, back when I worked for ....FORD.

Ended up being in a strange place. If the car sat on a little slant front down hill when it rained it would go into the right outside miror housing, run down through the plastic tube that the controls ran in, that tube ended right over the gas pedal. So the guy always parked in the same place and not a drop of water anywhere after a rain, he would back out of his driveway, start up the road, go around a right hand curve and ...the water would pour out on his right foot! The water was filling up in the tube and then when the car leaned going throught the turn it would come out. So they sometimes can be hard to find, frustrating for you and the mechanic trying to find it
tom