nn marks already read any messages which you have seen when you leave a group, unless you have set your defaults differently.
To find a message that you have already seen, use G to goto the group and when at the next prompt, answer
to see ALL the messages there. Alternatively, you might at that prompt answer
and then name yourself as the author of the message(s) which you would like to see.
Rutgers keeps most messages for at least 7 days, and in some groups (the local ones) for 60 days.
Enjoy cyberspace.
lc