My first thoughts require you and a friend to do a test. Get a friend, who's NOT familiar with the problem, lay their palm on the wall and walk along the wall for a good distance. Have them say 'bump' where they think they feel there's a 'high spot'. And if this coincides with what you SEE for shadows, then the problem lies with the mud-job. It wasn't properly feathered and sanded before being painted, hence, a high spot where adjoining sheets meet. They/you should be able to sense a high spot with your palm wherever there's a shadow. I have certainly seen this before, especially when the lighter paints are used. White paint, for example, will show this problem very easily!
If the aforementioned is not the case, the problem might be from moisture (as you stated). One way to test this would be to 'pick' one stud, prime it up with some latex Stain-kilz (or similiar primer), and then paint over the area in question. The next day, if the shadow goes away, you'll have your answer. The primer will prevent the stain from showing through on the finished paint. Do prime and paint at least 2' on EITHER side of the shadow. This way, if there's any difference between the paints you use, you won't confuse THEIR shadow with the one you're trying to solve! Use good primer and paint from SHerwin-Williams.
If there's any more you need, post up. But by all means, let us know how you make out. My best to ya and hope this helps.