Ehm… I guess this is the crux. He doesn’t use our Discourse instance directly much if at all, and relies on email. And since he never got an email… 
And since his “Frequency for receiving assigned topic reminders” is was defaulted to “Never” I assumed that was why. (Insert reminder of what happens when you “assume”.)
So… What I want to figure out is how many Staff will not receive an email when they’re assigned a topic. I’m expecting to figure this out via Data Explorer. I just need a pointer (or two) in the right direction.