In my experience, weird musty odors that show up in humid conditions but go away with a good airing out are caused by (ugh) mold.
Before you freak out, there's lots of different kinds of mold, 99.99% are not toxic (merely irritating), all houses have a little of one sort or another, and most of it can't be seen easily.
If it comes back, try to isolate the specific area that the odor is emanating from. Mold likes porous surfaces like wallpaper or wood or carpeting best, but can hide anywhere. Try spraying the area with some enzymes, they will seep in and kill the spores dead so they don't re-bloom.
The absolute best enzyme spray, in my opinion, is this stuff they sell for pet odors and skunk odor removal called "Nature's Miracle". A lot of pet stores carry it, and if you can't find it locally you can get it cheap from Amazon.