What does it mean when a device stops showing up on Android Device Manager? On 2 November I forgot my (brand new) Wifi-only Nexus 7 on the plane. Despite realizing it &lt;30 min after disembarking they didn't recover the device.

I have been regularly monitoring https://www.google.com/android/devicemanager in the hope that it would come online. The device showed up but was 'last seen on 2 November' (the day I lost it). However, when I tried again today to locate my device it no longer shows up in the list. I have checked https://play.google.com/settings and the device is still listed there with visibility 'Show in menus'.

Any idea what could cause the device to suddenly disappear for the device manager? Would that mean that it's re-registered? If so, any way to find out by whom?

Cheers,
Chris.
If the person, who got your device, gives it a reset(hard reset) and signed-in with another account(which is the default account in this case), then you might not be able to look up your device anymore. But if, he had not, then the possibility is that, he lives somewhere, where he does not have any wifi-access. Therefore you could not track him anymore.