Thanks for the suggestion! Due to the central AC running frequently, the humidity is low in the house, but we do see condensation on the windo was but, hmm, that would be on the outside. Maybe I should get the hose out and spray the outside of the house in that area, to see if anything happens on the inside... There is a bay window in the vicinity with shingles on the bump out.
Are you sure the condensation/humidity is low? I would ask around and see if someone can either rent or loan you a dehumidifier. You might be surprised. We were. We then looked at what could be causing it. Turned out we had a lot of air leaks and one leak underneath, small but enough to add to the moisture in the air, the house that was was causing all of the humidity in the house. Especially when the temperature went up and the water to started to evaporate. We also installed a whole house air and heating system which comes with a whole house fan. Next up was fixing the doors and replacing all the windows. We don't use the humidifier anymore.
Once the leak was fixed, we actually went out and purchased a few fans and put them underneath the house and had them going for several weeks. Had to dry out the sand. Huge difference. It was a tiny, I mean tiny leak, but it was enough over time to have the effect on the house and humidity.