smallest pinhole leak in the world?

The flat below ours has developed a patch on the wall that is showing signs of water damage. The building was probably built in 1830ish.

The patch starts at the base of the wall and is no more than 20 centimeters in width and spreads up the wall a bit for about 20 centi metres. There is another oval shaped patch about 10 centimetres length by 5 centimetres above this patch but the two water damaged patches do not meet. There is some racking in the plaster where the two patches are.

The patch has apparently been there for about 7 months and has not shown signs of steadily increasing.

The occupier thinks it is probably a leak in the plumbing. If there was a leak and it was the smallest leak in the world wouldn't it have caused more damage by now?

I don't think it is rising damp as I don't really believe in it. The patch is next to her bed with just 20 centimetres of clearance between it and the bed. There is also a radiator there.

Any ideas?

The reason I am asking is that the occupier is suggesting we contact our dodgy managin agents to have a look and I am scared they are going to say rising damp which we have no insurance for.

One of these will tell you if there's a pipe behind there - or you could "drill" in with a 3mm screwdriver for a feel.

dont be shy....knock the plaster off and have a looksy......

has this always been here ?

is the plaster in contact with the floor ?..should be a gap to stop dammp travelling up.....

If the agents tell you it's rising damp, perhaps they would like to demonstrate how it can form two patches, and also say they haven't claimed the cash reward from Lewisham council for showing them a case of rising damp at last.