Is there a skylight above the shower? Could be leaking and the water has found a way down the wall between the interior panel and the exterior panel through the fiberglass insulation. When it gets to the floor level it finds a new path under a joist to then appear on the floor. It's also possible that it is leaking in at the roof and wall seam joint as well. Finally, are there any screws that are on the exterior of that wall ( light fixture, awning, etc) that may be allowing water into the wall?
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