Tracking remote user activity

["Roger D. Vargas" <rogerdv () softhome net>]

I have an strange problem, some days ago one of my users complain about 
somebody who was tracking him and knew all the mails and sites he visited. He 
says he only exchanged mails (using his yahoo account) with that intruder 
(who works for a 3M branch in Mexico and says that they use such tracking 
system "to protect themselves from virus"). 
My network uses a linux server running squid as proxy and most users use 
different PCs to browse the web and compose mails (so, I can almost discard a 
possible trojan).
Can somebody give me an idea about such tracking software and how can I 
protect my users?

-- 
Roger D. Vargas
ICQ: 117641572
Linux user: 180787
* Tanto si piensas que puedes, como si piensas que no puedes, tienes razón *
Henry Ford