Opt in to the Scanner Appliance Heartbeat Check notification and you'll get an email when we discover your appliance is offline after some number of heartbeat checks (1-5). You can configure this in the appliance settings (just go to Scans > Appliances and edit the appliance).
We attempt to make a connection with each appliance every 4 hours to see if it's up and running. This heartbeat check lets you know whether the appliance is ready to perform scans and is able to get the most recent software updates.
1) Go to Scans > Appliances. Edit your appliance (hover over an appliance and select Edit from the menu). Under General Information, select the Notification option and the number of missed heartbeat checks before we'll send you an email.
2) Select User Profile below your user name (in the top right corner). Go to Options and select the "Scanner Appliance heartbeat check" notification option.
You can tell whether your appliance is online by going to the scanner
appliances list (Scans > Appliances). You'll see 
(Connected) next to your appliance when it is ready to process scans.
Select the appliance and check the preview pane to see the number of heartbeat
checks missed if any. If there's missed checks, it's possible the appliance
has come back up since the time of the last check. As long as you see
the
appliance is ready for scanning and there's no reason for concern.