Bob asked: "Andy, did you ever ask & get a copy of the offending messages?"

Once, I think only once, they forwarded the offending message.  I responded to them but never heard back.

As I say, in almost every case, the messages they send to us (to the +owner address) sound suspicious, as if not by someone actually wanting to be removed.  I think they just randomly send these "Unsubscribe me" emails to "+owner" addresses.  I don't know why but maybe they want us to reply to them, so they can get MY email address and add me to their spam list.  It seems unlikely but I can't think of another reason.

Andy