If you use arpwatch to track what is going on in your network you will eventually search for an IP or MAC address in the arp.dat file. Here is a simple bash script which gives a nicer output than just using grep and it also converts the unix timestamp to a more human friendly format:

#!/bin/bash

cat /var/lib/arpwatch/arp.dat | grep $1 |  awk  '{$3=strftime("%Y-%m-%d %H:%M:%S", $3);printf "%-20s %-20s %s %s\n",$1,$2,$3,$4}'