A while back I had a mystery on my hands. I wasn’t sure if someone had gotten thru my system defenses.
This discussion covers a lot of ground using only standard tools (nothing like the battery of tools that Kali Linux offers). The nature of the symptom (unknown IP reported by arp-scan) did not reach the threshold of requiring those additional tools.
I am sure that many will benefit from a review of the discussion posted here (Note: the title is somewhat misleading):