Drilling: what's underneath my subfloor?
Sounds like there could be some self-leveling concrete somewhere under there.

Self-leveling concrete is typically used to create a flat and smooth surface with a compressive strength similar to or higher than that of traditional concrete prior to installing interior floor coverings. Self-leveling concrete has increased in popularity as the degree of flatness and smoothness required for floor covering products has increased, with vinyl goods becoming thinner and floor tiles becoming larger, for example


It's possible the concrete is only in the low spots of the floor, so might not be visible (or extremely thin) at the vent you checked.
Since there's a basement underneath, if you can get access to the floor joists from below that might be a better way to stop squeaks.