Group permissions changed

[sf_submit () yahoo com]

Fairly recently I noticed my ftp client wouldn't list files in certain directories on my server anymore - so I ssh'd in 
(it's dedicated), and did a ls -aFl on the files, hoping to see what the problem was - here are a few of the results:

-rw-r--r--  1 larry  503   371 2005-02-25 08:36 head.php
-rw-r--r--  1 larry   48   873 2005-09-09 03:23 foot.php

I never set the group ids to 503 or 48, so I checked just to make sure - and no groups with those ids even exist.  Is 
there an exploit/tool that causes this, and should I be worried?

I checked the processes running, and everything seems to be OK - same with any processes connecting to the internet.

I'd appreciate any comments