damp at boiler flue

[szaadchim]

we had our boiler vokera 80 shut down couple of months ago,

just noticed month ago the wall joining the kitchen wall in lounge got damp and slowly increased and all black now , got surveyer coming to check

suddenly i thought even though my boiler is off it can be my boiler then i saw at the flue which is going outside has a damp and removed the wall paper and then can see that all black and damp there
started thinking that our boiler is off and heating is off but when we use the hot water tap, even though there is no hot water in that its just easier we found as its on left hand as we knew it doesnot matter as the water is cold on both tap, but there is hissing noise from boiler that water is coming from there or so , never thought about it but after reading some articles about combi and condensing etc , realised or think becasue its condesing boiler its got something to do with water and vapor or something like that , as boiler is off but i am still using hat water tap and its going through the boiler so its doing somethig to the boiler and which in truns sending water or vapor or condese in the flue or wall
any help would be much appreciated