Take it that you don't have a combi boiler which would indicate water loss by needing to be continually topped up, or a water meter which would show water usage continuously.
We only discovered that we had a leak in our system in one of the solid floors we have downstairs when we changed to a combi.
Presumably all of your downstairs floors are solid?
Our plumber located the leak in our system by using a sort of stethoscope ( picture a metal rod, with a spike at one end and an earpiece at the other). Once he had narrowed it down to a fairly small location he chiselled out a bit of the floor at a time and found the burst pipe. Ours was just weeping and was making nothing like a gallon an hour, so yours should be relatively simple to locate.
Good luck!