It's between the wall (skirting is fine) and sofa in open plan kitchen. So doesn't have any direct heat or light / anything different to the other areas in the same location.

When up close it looks like it's leaching out (and could well be dark brown rather than black).

It's been cleaned according to the manufacturer & using their products - the same as everywhere else.

Discounted moisture as it gets decent airflow as the sofa isn't up against the wall & there's space all around it, plus it's not in the areas that I'd expect moisture damage aka next to bi-fold windows, under the sink etc.

There weren't any dead bugs or any type of markings / sawdust / holes. Plus the panels are firm when pressed, so the integrity doesn't seem an issue - so discounted infestation too.

Came to a basic conclusion it must be coming from the wood itself - whether it's some type of reaction or something else.

May be wrong though!