Any thoughts on what could have caused/is causing this damp patch?

[Fodele]

It is at the lower corner of a window, usually covered quite tightly by a curtain so I have been able to ignore it for quite some time. I don’t think it has really changed much over that time.
When I first noticed it I thought it might have something to do with drying bedding on the radiator close to it, and not having much air flow to the area.

I bought a damp meter and according to the booklet it came with the very centre of the corner where there is some mould is just within the bottom of the range for ‘wet’ but the rest of the stain is fine.

I have attached photos of the external wall where there are some dodgy bits of pointing etc, but nothing obviously linked.
I don’t think there are any pipes in that wall.

What do you think?

Go out when it’s raining and look at the roof edge in that corner. See if it drips round the gutter board and down, soaking the brick.

That pointing could be contributing though. Our chimney needed pointing and water got in and leaked through our bedroom ceiling.

Thanks @NoShirtNoShoesNoSheldon. It has been raining today and you can see the line on the flagstone where it is sheltered by the overhang. It has been very light rain today, maybe different in heavy rain. I would have thought it would be more widespread inside of that was the case though?

I've recently had a similar patch (mine was more on the window reveal itself) which turned out to be caused by the sealant between the window and the wall having deteriorated, which was allowing water in. Looking at your pic, I can't see any sealant at all, so no idea if your windows don't need it or if that could be the problem. It was a very cheap and easy fix though!

There is sealant, but pretty poorly done and there is a big gap that more or less lines up with the area on the inside, so going to fill that as a first try! Thanks 😀

We had something similar, and it was because a gutter was broken and blocked.