In a similar vein to queries to finding/diagnosing Windows Servers and Unix servers, the following queries may help a customer to find/diagnose ESX servers that have not been scanned.
List all unscanned ESX servers
SELECT TargetName , HostnameOrIP , SuspectedOrScannedOS , FoundDeviceID , Reason , TargetSet FROM [history].[v_DiagnosticsDeviceList] WHERE SuspectedOrScannedOS='ESX' AND Status='Unscanned'