Damp patches on kitchen vinyl floor, concrete floor underneath. Any ideas?

[CrapBag]

We bought the house last year. When viewing in June I didn't notice any black on the kitchen floor. Surveyor said damp was reading as high on his meter so the EA got a specialist in but they said they couldn't be expected to lift the vinyl so wasn't really checked properly. Specialist said he wasn't picking up damp. Seller told him the damp was from an old plumbing leak that had been fixed. I don't believe this as the worst of it is no where near any appliances or plumbing at all.

We came over the day before completion and I noticed the black on the floor straight away. This was September. Since then it has got worse. We had to have the specialist back for some woodworm treatment and I showed him the kitchen floor and he said it wasn't as bad when he saw it before. I also asked him about this plumbing leak and he said he has to go by what he is told but I got the distinct impression he didn't believe it either.

Any idea what it is? The EA kept insisting that vinyl sweats and it fine etc etc but then she would say that. The kitchen is extended and the extended bit has a timber floor. This area is fine. I would say the flooring has probably been down for 8 years. House was built in the 50s so should have had a damp proof membrane (so I have been told).

Pull up the vinyl. Presumably there is a concrete or other solid floor underneath.

The water supply pipe probably runs under or in this floor. You can guess its position by looking at where it comes out of the floor, probably in the corner where the sink used to be when the house was built, and where the stopcock is, probably next to where the garden gate used to be when the house was built. If this pipe is steel, it will have rusted through by now, probably at the elbow where it turns up to rise through the floor. You may find a concentration of moisture here. If you turn the stopcock in or by the pavement on of off, in the middle of the night when it is silent, a young person with sharp ears may hear the hiss of a leak stop and start.

If the pipe was altered or joined when the extension was built, there may be a leak where it was cut.

We had this happen in our old house. I was told it was the concrete leaching. You could actually see it in patches on the attached garage floor, it would dry out to look like white powdery stuff.
We ended up tiling the floors in the bathrooms to stop the lino looking like it had patches on it. never had another problem after that (we had no water pipes in the floors)

Thanks for the replies.

I wouldn't have thought it was a pipe leaking. It is in many different random places and nowhere near where the stop cock is. I am fairly sure the sink is in the original place as all the houses around here are similar and all the sinks are under the window in the kitchen at the back of the house. Where this is is no where near the extended part. That's more over to the side.

I would love to have a tiled floor but given the size of the room, it would cost a fortune and where the original concrete floor meets the floor boards of the extension, you can feel a ridge so floor tiles wouldn't lay across it. We have been quoted £198 just for the vinyl for the room so goodness knows what floor tiles would come to.