You say a lot of water !! How much on the boilers pressure gauge ? 1 - 3 bar ?
The central heating side is a loop of sorts so like someone has already mentioned you wont be able to realistically eliminate certain area's of the house just by shutting down rads !! It really is a case of lifting up floors till you find it !!
If your only losing a small amount of pressure, i.e 1 or 2bar every fortnight or so, its unlikely it could be traced by using a damp meter through the floor !!! Its going to just drip to the bottom of the foundations nd eventually get sucked up by the ground ! (Causing no real damage at least !!)
As much as you would prefer not to, the only real way to do it would be to take up room by by room fella! Starting of course where you feel most likely !!!
Its a bummer I know but at least by the sounds of it you dont have concrete floors with the pipes inlaid !! Real nightmare time !!
Hope this helps a little.
Del (Builder & Plumber by trade).