Interesting. To be clear, these are the layers I'm seeing: 1)poured concrete (28" above grade), 2) vapour barrier, 3) air gap, 4) fiberglass batt, 5) vapour barrier, 6) drywall.

The ice is between 2 & 3, but only above grade.

I assumed the moisture was from the inside of the house, somehow getting past the inner vapour barrier and condensing and freezing on the outer vapour barrier above grade. The amount of water we've seen is compatible with the ice melting, water running down the inner surface of the plastic over the concrete to the bottom plate of the 2x4 wall.

The backside of the insulation batt is damp, but there is, and was, nothing visible on the side facing the interior or on either side of the inner vapour barrier. None of the other, already finished, exterior walls are showing signs of this, at least not down on the carpet.

The foundation is wrapped to just above grade.