Bought a house about 9 years ago. When I bought it, I was told new hardwood floors were recently installed. Overtime, the room closest to the air return has seen discoloring of the floors in the seams. It looks like moisture, but there have been no leaks and the damage is isolated to the seams. While the flooring has discolored, it hasn't softened, or at least not enough that it can be felt.

One of the pictures below shows the floor with no staining and the other shows the staining. Has anyone else seen this before? What did you do? Any other suggestions?

This looks to me like a possible leak under the flooring that is leaching upward. Is the flooring installed over a slab? Is there any plumbing near the location of the staining?

Flooring directly on top of slab. No plumbing in the area. It's been very slow growth with no evidence of water ever coming through the seams. The room is adjacent to sunken den, not sure if that matters. I thought it could have been a lack of vapor barrier between the slab and flooring, but I have no idea.

Honestly, I think you've entered the realm of "pull it up to diagnose the problem."