Overnight, one of my friends from elsewhere rejoined the site, and I woke up to this notification in my inbox (names replaced):
ThatFriend wants to be friends with YourCharacter! (5)
The key part being that (5). They'd sent friend notifications to multiple characters of mine which they know of, but all of them had been stacked up under that one friend request.
I'm switching through my character list to work out where the invitations were sent, luckily there's only a few likelihoods and I already know one of the five.