I came back to cut and install the cover for the lineal shower drain and found mortar had been dropped into the drain.
Lucky for everyone concerned I was able to take a long screwdriver and wet vac, chipped it into smaller bits and suck the chunks out.
And yes, I did have a cap on this when I was done. It was up to the tile setter to install the final piece and make sure he wasn't dumping mortar down the drain.