It use to be difficult to detect water intrusion in the walls of homes. To really tell if there was an issue you often had to drill holes into the walls. And Sellers were loathe to allow buyers do such an invasive testing when they might just walk away from the transaction if they found something they didn't like. These days some inspectors are carrying expensive, but very accurate, thermography meters to check for water intrusion.
These sophisticated devices use high definition thermal imaging for finding hidden water issues as well as discovering details about the heating systems. And it's all done without any damage to the home. The main company that manufacturers these thermal imaging cameras is called FLIR Systems. There are many other applications for these devices, and they sell them to a number of other industries, including the food industry, law enforcement, marine surveyors, just to name a few. They often cost from $15,000 up to around $60,000. So not many inspectors carry them. I suspect some inspectors may try to charge a premium to test with them. But at least one inspector here in the Seattle area uses them regularly without charging extra.