We had a slight discolouration in a small area of the bathroom ceiling appear a few months ago. It has I think been correctly traced to wind blown rain making its way under roof tiles, which hopefully is now fixed, but I have been keeping an eye on the area with a damp meter and an IR thermometer, just to ensure it is drying out.
Initially the meter was showing full scale in contact with the stain and nothing on the rest of the ceiling. Now it shows it is just slightly damper than the rest, except when the bath or shower have been used, but it soon drops back to its usual low value. My guess is that the tiny extra bit of moisture where it was slightly damp, makes the area cooler and therefore moisture in the air more readily condenses there. Checking it with the IR thermometer, the stained patch is a couple of degrees cooler, than the rest of the ceiling.