If the source isn't obvious, you have to consider that the source may be INSIDE the house.
For example, if your clothes dryer is in the basement, maybe it's leaking some of the 'hot air' into the basement. Also, it's possible that moisture from cooking can make its way into the basement. And thirdly, if you have a sump pump, it needs to be tightly covered. Every now and then, throw some water mixed with bleach into it. This will kill any bacteria.
If your basement is too tight, moisture will collect and mold/mildew will grow. You need to look for more 'signs' of moisture. Get ahold of some good lighting and start exploring. Other than that, I'd be inclined to think that the moisture is coming up from the floor. To test this, take a 2' x 2' piece of plastic and duct-tape ALL the sides, completely SEALING it, to the floor. In a day or so, see if you notice any condensation on the UNDERSIDE of the plastic. If you find it, then you've found a source!
My best to ya and hope this helps.
PS: If you can get ahold of a dehumidifier for a short while, let it run contiguously to see how much moisture is being generated in the catch bin. If there's very little, I wouldn't worry about it. All you may need to do is open a window from time to time. Or, run the dehumidifier when the need arises.