I recently had a home inspection in which an infrared scan device detected water below the master bath stall.?

Very interesting question. Infrared thermography is typically used to identify thermal loss (heating energy or cooling energy) in a building. The technology works by detecting temperature differences between surfaces. That’s why an uninsulated portion of your wall will show up on a scan: the wall is cooler there than the surrounding areas. FYI it’s best to look for heat loss in cold weather.

However, if conditions are right, an infrared scan can also detect water leaks. The equipment doesn’t spot the water—it spots the temperature difference between the water and the surrounding material. So if water in the trap was considerably warmer or cooler than surrounding area, I suppose it could show up on a scan. Water leakage during a storm might also show up because the cool rainwater would be a different temperature than surrounding areas. And if water in an area is evaporating (maybe from a surface saturated by a leak), this fact may also show up on a scan due to evaporative cooling.

However, sources I checked indicated that using infrared scans to detect water leakage requires a great deal of interpretation—it’s not nearly as straightforward as using it to detect thermal losses. So before you rip out a ceiling to find a leak suggested by a thermal scan, try to verify the reading by direct inspection or perhaps by using a moisture meter. Here’s some more information from This Old House about the uses of thermal scanning to detect heat loss:

http://www.thisoldhouse.com/toh/article/0,,20151396,00.html?xid=yahoo-answers&partner=yes

Additional info about thermographic scanning is below.