This Fall I noticed that my boiler had lost some water pressure over the summer. So I added about 5 gallons of water to bring the water pressure back up to the normal psi(20). Then I fired up the furnace to try to find the leak. After 24 hours I never found the location of the leak but still lost about 4 gallons of water. I then ran the furnaces one zone at a time to isolate the leak. Second floor was good(no water found). first floor was ok(no water). Then I finally located the leak (kinda-still no water) I think its under the foundation slab.

without ripping out the whole floor how do I find the leak? HELLLP!!!! Any Ideas?