Steve, It is (was, I hope) leaking from the seam where the curb meets the concrete shower pan near the wall and the seam at the top of the curb at the wall. I'm not a pro at this but it seems to me that the curb should have been incorporated into the pan and made of concrete at the time of construction. Instead the tile man built the two layer pan with rubber membrane, so far so good, but he left the membrane hang out the side of where the curb was to be, built the curb out of 2x4s then brought the membrane over the top of the curb. This may have worked to a better degree if he had not nailed the membrane to the shower side and top of the curb with a 1/2 million nails. Also, the membrane was not brought up the side of the walls at all before the cement wall boards were installed so there was some leakage there too, although surprisingly very little.

Whiplash,

I didn't have to redo everything; What I did was remove all the grout from around the seam at the bottom of the shower and at the ends of the curb. I let the grout dry for two days then put two applications of silicone sealer a day apart. Tomorrow I reinstall the hardware, getting pretty good at that,
and give it another test.

Wish me luck,

Jack.