strange output of who command

On one of our Tru64 systems, a DS20E alpha server with Tru64 5.1B  
pk5, we have an odd behaviour.
At least I can't recall I have seen it before.
To see what users are logged on, I give this command:
# who -T
user01    + pts/0       Nov 23 08:24
user02    + pts/1       Nov 23 07:51
user03    + pts/2       Nov 23 07:54
user04    x pts/16      Nov 22 15:45
user05    x pts/17      Nov 22 14:23

The strange thing in this case is user04 and user05. What does the  
"x" means in their lines? The who man page says nothing about this x.
These users have no processes on the system, so why are they listed  
at all?