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Take the screws out along the edge and pull it down and look into that area with a flashlight while running some water down the suspected drain. The shower P-trap should be visible from one of the service bays or from the side of the shower under the pan. Kind of an odd cause here. Please get back to me with what you find and we can go from there. All follow ups are included in our fee.
Just checking back in to see if perhaps you missed my earlier response to your question or if you need more input from me on this issue?