First place to look is the Apache's log files.
Go to the directory where Apache is install and
look in the file: access_log
See if any connections have been made from other addresses.
Sounds like a router problem.
So you'll need to check at a packet level or the routers logs.
use one of these programs on the Linux box:
You'll have to install them as they are not standard RPM installs.
Best way to find out is to dialup to an ISP outside your internal network and ping the IP address, if that fails due to a firewall try to traceroute to the webserver, as most admins don't understand how to turn this off.
If traceroute's last hop is the router then you know it's the problem.
Or use this URL to show you, replacing the http://www.domain.com
with the correct one: "will take a few mins"