Rising damp?

[bexta]

Can anyone help me?
I've had this patch on one of lmy iving room walls for a while now, I initially thought it was the plaster drying out. It is only a foot from floor to the top of the patch but is roughly 3 feet wide.
3 years later it is still there and its spreading. The house had been damp proofed/damp coursed before I moved in 4 years ago so I don't know how long ago it was done. What confuses me is that the patch is no where near an outside wall, the other side of it is in my kitchen.
I have checked all of the exteria for obvious leaks to the guttering and I have also filled up any holes and cracks outside.
Last night I noticed a few silver fish coming from between the floor and the skirting board, I hate any creepy crawlies so I have to get something done. Can anyone else suggest what I can do?

[Malagabob333]

Could it be a small leak on a waste pipe or a continued drip off a water pipe in the kitchen?

[bexta]

Thanks for the suggestion. I don't think it could be that, the waste pipe and the sink in the kitchen are on the opposite wall.
There is nothing on either side of this wall.

I had a thought actually, under my wood floor in the living room we have stone tiles lay on a bed of ash or soil of some sort. Could it be condensation coming from under the floor?