the idea that it's a pipe in the floor is a very fair one, especially if the pipes are buried in concrete. in most other cases, a leaking pipe or radiator can be detected because there will be a damp patch under or around it, sometimes on the ceiling of the room below. The infra-red heat camera can be very good for looking at a leak in a concrete floor.
It's possible to isolate sections of the heating section by putting isolation valves on the pipes leading to them, and seeing which section(s) lose pressure. I'd be inclined to try that and then just repipe that whole section rather than tinker around trying to find the worst bit. Often if one bit of pipe is leaking, all the other pipes of similar age and material, installed by the same person with the same techniques, will also be suspect. Unprotected metal pipes in a concrete floor are quite a problem. Renewing the piping may be quicker and less costly than a long (possibly fruitless) search.
if you are lucky the leaking pipe will be under a wooden floor and a hatch can be saw out for access. Some people resist that if they think their tiled or laminated floor is more important than finding the leak.
If a leak is on a hot pipe, such as on a boiler or pump, the leaking water may evaporate so quickly that you don't see a wet patch. however there will be a noticable mark, usually green or white, from copper salts and limescale.
You say the leak was not noticable until you started using the heating, so I'd suspect a metal radiator pipe that is moving with heat-related expansion and contraction.
Leak sealant additives are a bit of a bodge and won't help with a joint that is moving. Your new plumber sound to be better.
Your insurance will help with damage caused by escape of water, but I think not with tracing and repairing the leak.
Try bleeding a bit of water out of a radiator and familiarising yourself with the smell. Also try wetting a bit of old plaster, dusty concrete, and old wood. With practice you can often smell a leak.