I have a damp patch outside my house which has been consistently present for 6 months or so. There is no evidence inside the house at all.
The amount varies daily. It is located on the 3 bricks below the airbrick and spreads roughly along the damp course for no more than the 3 bricks below the airbrick. I have cleared away the earth to the foundation level to ensure that there is no water from the ground coming up. But still it comes and goes.
It does not venture too much above the dpc but it can do by 1 cm or so on a heavy day.
I am at a loss as to what this is, other than taking out bricks or ripping up floor boards.