It probably means that the individual has a different
address subscribed which is forwarding to the target which
is being reported in the D.E.M.  (My comment assumes that
the reported address is NOT actually subscribed but still
bouncing.)
 
GREP or SCAN for similar userid fragments or host fragments.
If that does not give you a suspect subscriber address, then
turn off monitoring for a day and let LISTSERV send you the
full bounce for further analysis.
 
/P