Here is a good trick for you to try. Take your laser thermometer (you do have one don't you) and shoot a spot a foot or two away fro the suspect spot then shoot the suspect spot. If it is damp there will be a difference in tepmerature. Usually cooler by a few degrees, but a hot water leak will be warmer. Bottom line is that there is a temp difference. This will not be as good as a moisture meter, but it gets you in the ballpark. For a moisture meter, I would recommend one that is non-invassive and can detect moisture behind items like tile, drywall, etc.

What you are describing can be accomplished with a flash light bulb (or LED), a couple of wires and a battery. While that will work with moist areas because water conducts electricity and completes the circuit, it will not appear as professional as using a moisture detector (or the laser therm.)

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