Moderation notification, but nothing in queue?

Do you have any other moderators/admins on the site that may have taken action on it? That's when I have personally seen something nothing in the queue after checking a bit later, but perhaps there could be other scenarios. Crossing off the easy question first!

Have you checked the "Possibly Abusive" and "In Process" tabs as well?

Yup, I did. And I confirmed that it wasn't another admin...or at least, not that anyone remembers or is owning up to. I took the opportunity to clean up our Administrator list. I'm not too worried about it. If it happens again, I'll report back. For now, I'm guess that and admin clicked something and just forgot or didn't realize.

FWIW, you can see who has approved or denied using the 'Approved' and 'Denied' filters in cases when nobody remembers or owns up

Nice! That helped. Mystery solved. No more "my dog ate my homework"-type excused allowed. ;-)