Water could be soaking into the cinderblock just about anywhere, but the most likely spot would be where the block meets the adjoining brick chimney, at the top.

Water migration is seldom a problem on vertical masonry surface. The typical problem areas are caps, rakes, etc, where water can collect in recessed mortar joints & have time to penetrate.

The first place to look is the hardest spot to get to, right up top between the 2 clay liners.