It could be moisture...or it could be a paint issue.
I have seen this before when the builder, does not use the same primer at all areas of the wall.
It looks like someone painted the crown moulding , then did the area of the wall around the crown moulding. Then painted the other wall area after they cut in around all the trim. This could be the issue, you just never noticed it.
This happens when the light gets different also. Like when you work different hours, or buy a new fixture, folks see things that where never there.
If it is water, is it actually wet?Did you poke it or test with a moisture meter? If you need to, get a pro in there to help you, you do not want this to go any further, Mold grows fast.