That's a pretty good sign there's moisture behind that area. Not sure if you can access from below or follow the plumbing but I would expect that you would start to see it expand pretty quickly. With the plastic plumbing in these walls, anything is possible.
I would buy or rent one of the pin hole camera's available from Home Depot or Harbor Freight, drill a small hole where it can be patched later, and inspect.
Good chance you will have to open the wall up to repair but inspect first just in case it's not the case.Edited by: r2millers
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