Damp smell from kitchen cupboard

[masamune]

When I open one of the cupboards in my kitchen (floor level) it has a damp/earthy smell coming out. It is a pretty dated kitchen from the previous owner.

I removed the back panel which has a 50mm void and noticed that the floorboards were visible. The smell is coming from the concrete slab right below. There are no evident pipes behind but I can't guarantee what is under the floorboards. Other cupboard on the floor don't seem to have the smell, at least not as strong.

There are air bricks around the perimeter on the ground floor, some of which were blocked and I cleared them. It is hard to describe the smell, it is not unbearable, more like wet soil though still annoying. I am not sure if there is any leak below, or naturally how the dust and concrete under the suspended floorboards smells. In which case I may just need to seal the bare floor?

One of the builders I used before told me to place a membrane and seal it but I am not sure what exactly I need to do. I may have access below the cupboard by pulling the lower face panel.

 

[Burnerman]

Ascertain there are no drain or other pipe leaks.....you may get some sort of clue by taking the cupboard plinth off, but obviously you still can't see under the floorboards.
What's the possibility of lifting a board or two in front of the cupboard for a check?
There's a good chance that you've helped the problem by opening those air bricks but I'd still prefer checking it out.
If all seems ok, then seal up with expanding foam between the boards and the wall behind the cupboard.
John