Hmm, it certainly looks like something heavy had been there and then removed, as it's much too sharp an outline to have been from someone kneeling (which was what I thought too at first). But my understanding of these photos is that they were taken immediately after her body was found, so whatever had been there was already moved before the cops starting collecting evidence. I'm sure that eventually I'll remember what makes a shape like that, because it seems familiar.
Remember that the round part may be the mark from the object and the wide, long part may be where it was slid across the floor.
There were paint cans reportedly found in that room, too, and the sound of paint cans being slid across the floor will also produce the sound of metal scraping concrete.