While trying to see if my kindle was in the house or not, I got to looking at the arp table, and specifically:

ip -s neighbor show

Which prints out the table along with some info about the entry being stale or reachable and a few numbers. The man page says nothing, but luckily stackexchange came to the rescue:

After a look at iproute2 source code, the fifth fields gives timer information on ARP cache entries:

• X/./. : Number of seconds since the ARP entry was last used
• ./X/. : Number of seconds since the ARP entry was last confirmed
• ././X : Number of seconds since the ARP entry was last updated

Those timers are notably used to manage stale ARP entries and decide when a new ARP request should be issued. Refer to this insightful answer for more info about ARP age timeout.

https://unix.stackexchange.com/a/474006

Perfect. So I know my kindle was last seen 20 days ago, which isn’t all that helpful, hopefully it’ll turn up in the house somewhere 🙂