We just bought a 3-year old house and there is an odd smell of moisture in the master bedroom and attached bathroom. It smells like someone just took a shower although the house has not been used in 10 days. During those 10 days, the central air conditioning was set at 80 degrees. We can't locate the source of the smell as you get used to the smell pretty quickly. We noticed the smell during our visits before buying the home, but it seemed like it could easily be coming from their items or having just showered. The master bedroom and bathroom are on the 2nd floor of a 3-story townhouse.
It sounds likely that you might have a minor leak somewhere perhaps. You haven't really mentioned where you live as that can have a big impact on the expected environment in the home. If you in a low lying area and have a basement it could be related to ground water in the area seeping into your basement.
More than likely though you simply have a leak somewhere. I've seen thermal imaging solutions advertised as a great solution to this but have not personally used a service like this. Here is a link to a site that details the process pretty well so you can get an idea of how it works. You'll have to call around your local area and find a company that offers these services though.
Once you've fixed the source of the problem and know the extent of the area affected your best option for clearing up related problems is going to be a dehumidifier. In most cases you want to aim for 40-50% humidity at most.
If you live in a humid environment and the ambient humidity is upwards of 60% inside the house regularly than that may be the sole cause of your problem. Either way your first step is going to be finding the source of the problem.
Maybe you could better describe the smell. Is it the smell of soap, cosmetics or just a musty wet odor? Regardless, I'd be checking the bathroom exhaust fan to see that it is properly vented to the out of doors and not into the attic or some other indoor space. I would also check the AC filters and clean the air ducts, both feed and return. Carefully check the adjacent areas, attic, basement, air ductwork to try to find a source of the strongest odors.
Also check the drains/traps as a source of your smell.