It's a fairly new house (about a year old). The concrete basement foundation walls are framed (wood), insulated, sealed with a plastic vapour barrier, but not drywalled.
I am led to believe that, during construction, the builder didn't immediately seal around the basement windows before he installed the insulation in the basement. Subsequently it probably rained and moisture got in through the unsealed basement windows. Now, I can see the condensation on the inside of the vapour barrier i.e. the side closest to the insulation. The vapour barrier is dry to the touch from inside the basement.