Strange damp patch on middle of internal inside wall ?

[chuzzwuzzer]

Hi. We have a 1930's semi. Beginning of summer I noticed a damp patch on an internal inside wall in our kitchen. Only noticed it because the paint had slightly bubbled. Thing is this is a dividing brick wall that seperates the kitchen and dining room and on the other side of the wall in the dining room its bone dry. The damp patch starts 18" from the floor and is just under 2ft long. I have taken the bottom plinth off the units in the kitchen to see if theres any damp below and its bone dry. Obviously not coming from above otherwise it would be all down the wall and not coming from below. No pipes in the walls. Thought perhaps condensation but we've had a pretty decent summer and with the doors being open most of the day so wouldn't have thought it would have gotten worse but the patch seems to be getting bigger. Any ideas what could be causing this and how do we rectify this. I've highlighted the damp area in the pic .
Cheers.

 

[John D v2.0]

Does look like an ideal spot for stagnant air, but if you've kept it ventilated so well it's surprising. Try wiping it off and tape some cling film over it and see what happens.
Also get a cheap humidity meter/thermometer off eBay. I was surprised how often I thought it was muggy and actually the humidity was low, and vice versa.
Edit: and an IR thermometer is also useful to see which parts of the wall are coldest, for me that exactly correlated with the damp/mouldy areas.

 

[chuzzwuzzer]

We do have a dehumidifier running in the hall when its particularly damp in the colder months and we tried using it right in front of the patch for over a week with the damp just reappearing. Borrowed a damp meter and it shot right up but a few mm either side of the patch was dry. Thing is we have a very similar patch on our chimney breast in the front room which is about the same distance from the ground.