I like to use a freeware called Look@Lan. I can see every device on the network and it's ip address. With some fanangling, you can get the MAC address as well. With her problem occuring sporadically, you could also launch Wireshark(another freeware) and capture packets, etc for a while, then go back and search for that ip address and see what all has it and what responded with it...but you seem to have already figured it out.