Initially I scoffed, but then I recalled that certain RF bands are disproportionately absorbed by water and this is used for certain sorts of atmospheric imaging by satellites. So you'd need a moisture "light" and an RF camera. Other than being cost prohibitive it sounds like an awesome toy.
It still works pretty well in the IR band. I know that one from experience due to storm damage to our roof ;-)
A leak only turns invisible if the water has the exact same temperature as the wall and there is no meaningful evaporation happening (as that cools the affected area).
Of course don't let me stop you from actively probing your all using RF. Though also there you might have good chances with IR, since wet $stuff should behave differently than dry $stuff ;-)