Basement floor divots

Replacing the dryer in a house we bought a few years ago, and noticed these "divots" in the floor of the basement under the old unit (they are actually raised from the floor about 1/4 inch). Don't appear to have anything to do with the dryer itself but curious what they may have been for. They are about 24 inches apart, quarter for scale. They are near the electric panel, plumbing for the washer, and a pit toilet if they may be related to those things. Any ideas?