We have come to the conclusion that Site24x7 is using the resolved name of a device rather than ping as the basis of availability.

When we have investigated, in some cases we have seen that the device has remained up yet the network device (usually snmp based monitors).

We discovered this when we had an outage on a Domain Controller  server at exactly the same time of the outages.  

Is this a known feature? 

Surely a ping test would be a better option to determine if a device is contactable.