We've got loads of marks like this around the house, this is a big one, but generally they're little holes from where plugs and screws have been taken out of the wall. Last owner has clearly tried to fill and sand and paint over but it's not covered it.
My guess is just a few layers of paint, but don't want to waste my time waiting for a million layers of paint to dry if there's a smarter solution. Plus they've obviously tried this and it's not worked
It's going by through to plaster board I think as it's a new build and so we're getting s bit of a papery texture under, do I'm reluctant to sand anymore anyway!
Thanks!!