Spots on wall

[Tulipa]

Hello, does anybody know what these spots or stains on the walls are please? Is it some kind of mould? What's causing it and how to get rid of it? I guess it might be hard to see they're not dark looks as it is wet but it's round in shape, I'm not sure but doubt that water marks would look like that.
Thank you.

 

[opps]

I may be damp, or it may be condensation. An easy way to test is to tape some bacofoil to the wall. If the surface facing you shows signs of moisture then you know that it is condensation. If after a few days, the back of the foil is damp, then it is more likely to be damp.

This is an exterior wall, I guess? is it rendered on the outside?

Has this happened recently?

 

[Tulipa]

Thank you, I might try that. Yeah, all on exterior walls. I'm not sure about rendering. Front of the house is stones and the side seems smooth.

 

[sparkwright]

Exterior walls.

Sometimes if there is rubble in the cavity, moisture or damp can track across the rubble and cause this effect.

 

[Tulipa]

Thank you, so it is a bit more complicated then. Probably not much can be done about it as it is a rented property I guess.

 

[JohnD]

Stand back and take a wide pic of the whole wall to show the layout

And the wall outside, including drains, gutters, downpipes.