After an hour or so, the window in question was bestowed upon me. I swiped it out of sight as per ganbustein, then went back to System Prefs and unchecked "Automatically check for updates" again. Five minutes or so later, the notification reappeared; this time I clicked "Details" and let the Mac App Store show me the specifics.
My thesis is that swiping it out of sight was like hitting the snooze alarm: even though I subsequently disabled automatic checking, that particular notification was already at large in the system, and nothing short of clicking one of the two buttons would dispense with it.
I guess I'll have to wait a week to be absolutely certain.