Yellow stain behind a bookshelf

[dipsyzum]

I removed bookshelf to give it a good clean and reorganise the room a bit and noticed yellow stain on the wall. I am not sure what has caused this or how old it is as the shelf has been there for over five years. How can I go about investigating it? Thanks

 

[cross thread]

See what happens now it has som airflow around it now the cupboard is out of the way

 

[Murdochcat]

What is on the other side of the wall?

 

[Nige F]

Have yo got a cat ?

 

[dipsyzum]

There is combi boiler in a cupboard next to it but the water pipes are going up and outside the wall (to the best of my knowledge). Not sure about the gas pipes.
I don't have cat.

 

[dipsyzum]

What is on the other side of the wall?

Other side is the side alleyway. No noticeable leaks or stains on that side of the wall.

 

[JohnD]

Looks like a water stain from above. Is there a bathroom upstairs? Or a pipe in or on the wall?

It does not look like a condensation mark.

Show us pics of the other side of the wall, all the way up to the roof and gutter.

 

[Murdochcat]

Other side is the side alleyway. No noticeable leaks or stains on that side of the wall.

How does the pointing look outside? Anywhere above the point and what about the guttering, if there is any