I actually don’t know if this is notifications leaking, digest-like summaries or what. I’ve asked one user to forward me one of these mail and will set up a normal user and investigate a while.
There is never enough time to investigate but maybe I make an effort this time. Earlier I’ve encountered notifications leakage as an admin, a few times getting a notification about topics I have nothing to do with (and possibly even have muted). It could of course be possible someone has mentioned me and then removed the reference and that has caused it.
And now, while messing around, I think I found something that might be a bit useful. Changing user preferences “default categories muted” could remove some of the problems for new users until I figure out what the users think is excess.