If it is being caused by damp or water dripping somewhere wouldn't you expect the IR readings to be lower than 200Meg? I've come across damp problems twice before, once in my own house where a leaky shower was wetting floorboards close to some down lights. The IR reading in the morning not long after using the shower was below 1 meg, whereas next morning before using shower it was around 20meg, presumably because it had dried out.