Another aphorism of system administration
Noticing when something shows up is easy; detecting when it goes away is hard.
Like all aphorisms, this has exceptions. And if you want to see it that way, it's a corollary of an earlier aphorism.
(An aphorism brought to mind as I contemplate our DHCP configuration files and wonder just how many of those Ethernet addresses are currently mouldering in a dump somewhere.)