Curious concrete

The lounge carpet had a curious dent that needed investigation (and the skirting boards needed exposing for painting) so the carpet has been rolled up to see what is going on underneath - and there's a curious concrete patch visible, with 2 holes, shown in the first 2 images.

We can't fathom why - any ideas?

The floorboards are ok. Just a few nails and staples to remove and it all needs a good hoover one I've stopped making a mess filling all the gaps, cracks and scrapes with Polyfilla.

The underlay is a good quality and seems to be fine to put back down. There's a few dents where furniture has been pressing in it, but they don't show when the carpet is down.

The electrician will be back next Wednesday to add a couple of couple sockets in this room.

There's a bit more filling in on the walls left to do by me, then the plasterer can get on with a gap above the lintel when he comes back. Then lining paper will go on and then I can paint.

It feels like good progress has been made this week.