I had posted earlier regarding high humidity and noted that I was pulling a lot of water out of the air (about 9 lbs per hour of running time), but household humidity ran in the low 50s most of the time.
The basement humidity was found to be on the high side (about 70%), and since the (tight) windows are kept closed, this was assumed to be the source.
Thanks to Dash's 'trick', my attention was drawn to the return system and found some large, hidden, leaks.
I've sealed the leaks and my 'telltale' indicates approximately neutral leakage. Since then, the humidity in the basement has risen considerably, while the house is about the same.
Would my assumption be correct that 'stack effect' is pulling moisture into the basement, and since it is no longer being drawn into the return ducts, it is 'collecting' in the basement (% RH rising) and into the house, since the RH is not lowering and I'm still pulling a lot of water out of the air.
My basement has a full perimeter french drain which feeds a sump pump. The drain is always bone dry and the sump never runs, except when I test it.
Will be examining foundation and sill plate.