What is this on the walls?

[purple099]

On pre exchange viewing saw this on walls after mirror had been moved.

My brain is freaking out in case its penetrating damp. External wall and partial fill cavity wall

Anyone got any ideas?

Wall wasn't damp, wallpaper a bit bumpy with what felt like grains of rice underneath wallpaper (20 years old or so).
Wallpaper in other rooms peeling at the top but no musty smell or

Survey said damp was within acceptable levels and only minor efflorence, but that was a few months ago so may have crept in over winter?

Any ideas?

Have you tried wiping it off? I’m not an expert in anything - but it looks like the mirror has been taken downwards, so any dust collecting behind has been dragged down across the wall paper. Or maybe they tried cleaning the mirror and over sprayed?

As above, I would try wiping it off with a damp cloth. On the other hand it could possibly be a bit of condensation that has collected behind the mirror - this is not uncommon, but, tbh, that would look like a dark mark. It certainly doesn't look like penetrating damp anyway.