Plastic shower drains occasionally leak, and these leaks are extremely difficult to find. A big stain on the ceiling below the shower will show up but leak may not be exactly there. Drains can be leaking only when actually standing in the shower, or tub, which causes the shower floor to bow just enough to make the drain connection leak. Drain leaks allow water to sneak around the outside of the drain where it’s connected to the tub or shower.