Mystery baseboard heat water leak.

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?

When I have to locate a leak under the slab one of the easiest ways is to turn on the boiler let it heat up for a while then take a wet mop and dampen the floor .. The area that dries first is normally where the heating piping is under and the faster drying areas are normally where the leak is

Great thanks. but I have a finished basement. I'll give it a try where i have no carpet. Thanks!

Should work as the leak will cause a warmer sop thus the water on the surface should dry faster .. You may need to mop it several times to pin point the most heat loss .. good luck