Here's What Happened With the Streaks

Don't you love inspecting over the Internet using someone else's description and a photo? Good comments. I think C. Wilson said it most concisely. Things like unconditioned space below, in a warm, humid climate with AC and maybe KDBT sleepers all possibly influenced things. The bottom line is that the vapor barrier was in the wrong place. It was above the subfloor, not on the concrete as it's supposed to be.  Here's another photo.

Actually, there were one and a half vapor barriers in this job. The rigid foam between the sleepers was a non-continuous vapor barrier. And the blue foam pad above the subfloor was a continuous vapor barrier. The slab moisture pegged my Tramex CE. Moisture from the slab wicked its way up through (or beside) the sleepers, and wetted the subfloor from below. Aren't the rules to either put a vapor barrier on the slab, or use treated sleepers and put a vapor barrier over the sleepers and slab.

The floating hardwood floor showed no symptoms. (The hardwood was being replaced at a purchaser's request prior to occupancy.) The vapor barrier foam pad kept all the moisture from reaching the hardwood.

After much consternation, the architect and engineer(s) agreed with me that the design was wrong: The vapor barrier was in the wrong spot and should have been below the subflooring, or some other fundamental design was needed.