I wrote
 
 : On one of my lists, a user tried to "unsub" but was told by Listserv that he
 : is not subscribed.
 :
 : As listowner, how can I see who the concealed users are? He thinks he
 : might (inadvertently) be one of them, though this sounds unlikely to me.
 
It turns out that he was indeed one of the concealed users.
 
Thanks to all who responded.
 
-_Andy