Can't see it matters much in practice whether its real or fake/mistake
I was in Okinawa when (the last but one?) real one went over. There were alerts on the media and sirens.
I wasn't clear, though, what one was actually supposed to DO with an IRBM alert. I was in a wooden rental cottage with no basement.
Hide under the tea table I suppose, which in Japan probably involves an elaborate ritual with graceful, formalised gestures and exquisite carpentry.
In practice I did nothing, just made myself a cup of tea. Maybe the Okinawans are better prepped, but I doubt it.
OTOH, one would hope the sizable US bases have a plan, though it probably wouldn't involve much carpentry..or tea.