I thought about the koban but only after the fact. Next time I'll try to remember that up front.

I don't recall any ID card accompanying the badges that were shown to me, though that doesn't mean there was one. My entry stamp was only a day old.

It was really just the slightly clumsy/incorrect way my passport info was written down that made me think it might not have been a legitimate stop. I'll run it past my Japanese colleagues at the office tomorrow and get their input. I'm in Yokohama for a couple of days and could head back to Kawasaki if warranted.