Site Supervisor II was here to take a look at the bath tub. He suspects some kind of fracture took place before the bath was installed - perhaps the tub was dropped at some point - and that it's taken this long for the rusty color to feed up through the enamel. He is baffled and horrified. His first thought is to speak with a cast iron guy he knows to see if the latter has any experience of such a thing happening.
We are hoping that it might be possible to find a fix that involves (1) patching the tub underneath to ensure it will not leak (and the bottom of the tub is perfectly accessible because the side panel is held on with magnets only); and (2) refinishing the enamel. Otherwise we are looking at pulling out the tub, the tub slab (our beautiful Vetrazzo) and all the tile around the tub. A major major pain in the neck for all of us.