Tub is a Kohler Maestro cast iron drop-in, white. It sat outside my plumber's shop for a month or so in its crate, then we moved it to the job where it was inside, uncrated, and in perfect condition--I checked it carefully (I'm the GC). We installed it by lowering with an air bag due to a tight alcove and no way to manhandle it.
Owner uses it once, to give the dog a bath no less. Later on I am crawling around in the bathroom doing some caulking and I notice a tiny little "spider" of rust in the bottom. Very small but it's there, and it's under the glaze, it won't rub off. My assumption is that the glaze has a pinhole or some crazing at this spot and the first use of the tub has caused the casting to start rusting.
I assume we need to remove this tub and replace it. If it were acrylic I might hope for a patch in place, but it doesn't seem likely with a glossy enamel item like this.