unknown user and group ids

[Warren Lamboy]

I am responsible for a RedHat system running kernel 2.4.21-32.0.1.EL.

I am curious about something I am seeing on this system, namely that
when I do an "ls -l" on some directories I am finding files and
subdirectories
that have owners and groups that I did not set up and whose ids do not
show up when I look users and groups with Red Hat User Manager
(/usr/bin/redhat-config-users).

For example I have some directories owned by userid = 608 and
groupid=20012.  Neither of these are valid according to Red Hat
User Manager (with no filter on the user and group list).

Can someone tell me how these ids might be getting assigned to files and
directories on this system?  Is there a way for me to do a general
system 
search for such ids?

I raise this issue because some users seem to be having trouble
accessing the directories and files having this owner and group
(and other "invalid" groups and owners).

Thanks for any insights you may have.  

-  Warren Lamboy

P.S.  In case it matters, I should mention that many such files and
directories are ones that some of my users have downloaded
from Internet sites.