Where is my leak?

I've got a new shower. It's a solid surface (so, like Corian, half an inch thick). The solid surface walls come down inside a flange on the solid pan (and then it's sealed there with silicone).

I'm getting a small amount of leakage that seems to be coming from the back of that flange. I opened a hole in the other side of the wall and the backing board behind the solid surface is dry. (The backer board ends just above the flange.) The plumbing connections are not leaking. So it seems...

Where is my leak?