Hmmm, I’m not a plumbing expert by any stretch, but that looks like a leak from above rather than from below. i.e. a pipe somewhere is dripping and tracking underneath the lino from the edges. You’re going to have to whip the kickboards off underneath the units (they should just pull off…held on with spring plastic clips) and see where it’s coming from.
Likely to be quite a slow one somewhere.
EDIT: too slow 🙂
The good news is that if it’s a leak from above, it’s not going to be the end of world but the hardboard is going to have to come up as well. 😐
EDIT EDIT: And who nailed that bloody hardboard down. 😡