I went back and checked -- there really doesn't seem to be anything on top of the concrete unless it is paper thin. Maybe it was stained, or maybe it's residue from the mastic?
You can tell because of where the carpet tack strip was removed.
I'm not really sure what "3 dimensional scraping" is supposed to mean. I think it just means razor scraping.
The top of the slab is actually a few inches above the ground, and all the documents we have refer to it as "slab-on-grade". It also isn't level -- it was built with a grade in the slab from the center to the sides of the house along one axis, about 1/4" per foot. In addition, due to settling it deflects slightly more near the sides of the house. Leveling it out completely isn't really an option because it would raise the floor too much at the walls, but the built-in grade doesn't bother me.
We initially thought about the floating floor but the flooring company we talked to said that in the long run a glue down would be better. They mentioned that a floating floor will stress the T&G system too much and slowly work itself apart over time.
Any thoughts? Reconsider the floating floor? Thanks in advance.