Hopefully you will have discovered the culprit with the Central Heating losing pressure.
One thing to note (In my opinion
) -
If you have copper pipes rising from the floor they should be mechanically separated from any tile adhesive and or cement / floor screed.
Modern copper is woefully less able to deal with the acidic nature of most cementous materials and as such will disintegrate if in contact with said cement over a relatively short period of time.
I have seen 22mm copper literally eaten by corrosion and pinholed quite badly after 4 or 5 years when laid unprotected in both concrete and floor screed.
Generally plastic pipe is much better in these areas but failing that there are products such as petroleum based Denso tape which will protect copper pipe and I have also seen good results with simple aluminium foil tape applied to the copper (I have no idea on the makeup of aluminium foil tape adhesive !)
If the CH pipe is leaking at or under the floor you may be looking at very small pinholes which will be difficult to see, trust the evidence when you lift tiles. if the floor substrate is properly wet then I would opine a 99% chance of a leak even if the pipe appears not to be leaking.
And finally have you felt around the bottom of both radiator valves?
If in doubt run a dry piece of toilet paper tight around the nuts which connect to the pipes, often this will reveal a small damp spot on the tissue even though there was nothing visibly leaking. Again this is 20 years plus of experience talking. Many times over my continuing career have i attended "damp" problems only to locate and sort previously unseen simple water based issues.