I went to view a house yesterday and came across a strange damp patch half way up the lounge wall.  This is an internal wall.  The other side of the wall is the dining room which is half wood clad.  What could it be?

No.  Both chimneys (in damp room and next room) are on different walls.

Not likely to be a buried pipe that high up, is it?

The house was built in 1830, the wood cladding looks old, certainly not that old, but not new.  Solid walls, as you'd expect, given the age.

I guess it could be grease on the wall that's been painted over.  Earlier marketing pictures don't highlight the patch so much....

It's a potential BTL, and fear this problem could grow arms and legs.