Can anyone help identify this darker patchy issue

[Cassas00]

I painted this bedroom. It's had two coats.

Most of its dried a consistent colour but the one corner above the radiator looks darker and patchy. The picture is 24 hours after the second coat. The first coat looked patchy in that corner too so I left it a day between coats too.

The room was previously painted all white.

I'm thinking possibly water stains bleeding through?

I'm thinking of painting that section with a stain blocker before repairing that one wall.

An my thoughts?

[Cassas00]

Just to add the wall is the one between terraced houses. There isn't any pipes above either.

[stevei]

What does it look like at night with the light on?