how to find hidden host within LAN

[desant1 at tin.it]

Hi everybody
I'm using RH ES4 with iptables as gateway/firewall for my 
LAN.
In the last week i notice in the iptables logs that a host within 
my lan is doing a lot of traffic.
The destination/source address of the 
packets and the used port suggest that this host is using peerToPeer 
application (emule or similar).
The problem is that i'm not able to 
identify this host within my LAN:
I can see his IP address (192.168.x.
y) and i can find his mac address througth ARP, but i can't ping it and 
there is no host within my lan with this Mac address.
I can't 
traceroute it.
Can someone help me to find this hidden host?