Advice needed. Swollen floor in bathroom worsening. Leak or humidity?

[Jotch Bob]

Hi everyone.

I recently posted about the flooring in my bathroom. Previous owner states there was some water damage leading to swelling of the floor.

I have noticed over the past few months that the swelling is extending, particularly along the seams. I am not very good with DIY, so I am wondering if this is humidity/condensation getting in under the seams or a slow leak. There is no extractor fan in the bathroom. The bath is rarely used. The floor still seems sturdy under foot.

There has not been any surface water and the ceiling below the bathroom looks fine. No bad smells or anything. I have noticed some browm, sticky material coming from between the tiles. Pictures attached for reference (most recent 2 on left, right approx 4 months ago)

Many thanks for any advice/expertise.

 

[CBW]

Looks like a leak to me, and if nothing else showing, I’d say a small one. As for the bath being rarely used, the pipes to it will still hold water, and any feed could have a leak.