Mmmm. It looks historical. If it was still wet, you'd have seen it coming all the way through to the surface.
This.
Could have been a bath leak from decades ago that wasn't dealt with well.
Draw pencil marks around the damp and leave it for a few months to see if they get worse.
@Howard started
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Mmmm. It looks historical. If it was still wet, you'd have seen it coming all the way through to the surface.