Security & UID, GID?

[Art Lemasters <alemas@rmi.net>]

     One account on my system (e.g., one user in the /home directory)
has had its group permission changed to from x to s without my doing
so, a couple of times.  For example, in the /home directory, one user
directory permission looked thusly:

drwxr-sr-x   16 user  user  1024  Sep 29 18:00  user

     I did not manually chmod the permissions that way.  What
might have caused this, and what are the implications, anyone?
Thanks for any leads on this, and yes, I have changed that group
permission back to "x" each time this occured.

Art