Flooring – What could be the source of this moisture

First, I'd redirect the bathroom vent out of the home. Best place is on the gable side of the house, but straight out the roof would also work. Depending on how much condensation you're seeing, and where it travels after condensing, this could be the cause.

Then I'd do some detective work. Start removing the insulation on the other side of the mold. Look for signs of water damage and follow them back to their source. There could be lots of causes, including a leak in the roof, plumbing problem, a clogged hvac drain line, or condensation on uninsulated hvac ducts. Water can travel a long distance before it finally settles on a low spot, so search far and wide.

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.