Hello, we've just redecorated an interior wall. Didn't notice any problem before, but now we have a darker patch in 2 places, which is blotchy in appearance, just above skirting board. The concrete floor seems dry, as does the skirting board. There are no pipes nearby. What could it be?! Our plumber suggested painting with gloss undercoat - he said if it is a stain it will cover it up, if damp, it'll still come through, so we will know if we have a problem. Any advice or suggestions?! Cheers