This appeared a few months after a termite treatment in slab construction. I thought you folks might appreciate seeing this, as the treatment may be common:
Holes were drilled every 12 inches around the perimeter for the chemical injection. The holes were filled with standard non-expansive grout. The installer said of this ‘oh, they pop out all the time’. ‘If you’re doing any sort of construction and banging around they come out’.
So I banged on a few more with my foot, and they popped out. The rest yielded to a light tap with a chisel. The typical plug appears about 10% smaller than the hole it is in, and it rattles if you wiggle it.
This is not water tight, not radon resistant, not chemical resistant and most importantly the gaps are big enough to allow the entry of termites (the termiticide used is non-repellent). See also here. Anything over 1/32" is a termite problem.