I've never used it to look for floor heat pipes, so take this with a grain of salt...
When I had my (existing) house inspected before I bought it 5 years ago, the inspector used his digital FLIR Systems camera on the walls and ceiling, from the inside. It was a cold day out (around 0 degrees F) and by the time he used the camera, the gas furnace had been running for 45 minutes or so. It could "see" every stud in the exterior walls and most of the joists in the ceiling upstairs... this was only through 1/2" drywall but they showed up very well. His could take still pictures, like a regular digital camera... I think some newer ones can also take video. He mentioned that he also used it for electrical inspections in commercial buildings and factories - looking at the wires in breaker panels to make sure nothing was getting too hot.
These cameras cost many thousands of dollars each, so usually you rent them. FLIR Systems also has one that plugs onto the back of an iPhone for $350; I've never used that one or seen it used, so I don't know how well that works. Some home inspectors or maybe insurance inspectors might have one, that you could use for a fee. Sometimes the local fire department has one, but I don't know if they'll bring it out for a private business. (Asking is free, and you already know their phone number!
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I hope this helps!