@KOM:
Are you sure this is what's happening? Squid, when running on transparent mode, will intercept all TCP 80/443 traffic and that's it. Inter-LAN traffic doesn't even hit the firewall unless it's travelling between different interfaces. I've been running squid & lightsquid for literally years now, and I've never seen anything like what you describe. What are you looking at that makes you assume squid is even involved here?
My squid is not on transparent mode, I've run a group policy management to distribute proxies into the domain devices. Well if that has any effect on the monitor then it seems that we have a major problem.
If not, well I guess C0RR0SIVE might be right, I need to re-check my network schema.