What are these holes?

[Wade Sikkink]

We are rehabbing an apartment in a building we recently bought. When we pulled up the old carpet and pad we found these holes all around the edges of the rooms. This is a ground floor unit so the subfloor is a concrete slab. They are in all the rooms. Some are open like the first two pictures and some are filled in like the third picture.

I don't have a clue what they might be. Any thoughts?