Hi,
that´s correct: the shortcut is "CTRL + ALT + F12" to disconnect the session via keyboard and is enabled by default.
You can also disable that function via a customized profile on the MMC.
Minimum Firmware Version needed: PCoIP Firmware 4.0.1 to get the shortcut.
Add: If you are using the Imprivata SSO Integration on your ZeroClients, you can disconnect the session by pressing F4.
That´s the "Imprivata default" 
Greetz, Jo!