In case you managed to screw up, somehow, the membership of All Unknown Computers collection (like me!) here is the query that will bring back:

x86 Unknown Computer
x64 Unknown Computer

sub-collections for all of your sites! In my case All Unknown Computers was empty, completely messing up my OSD deployments…

Rescue query:

SELECT
    SMS_R_UNKNOWNSYSTEM.ResourceID,
    SMS_R_UNKNOWNSYSTEM.ResourceType,
    SMS_R_UNKNOWNSYSTEM.Name,
    SMS_R_UNKNOWNSYSTEM.Name,
    SMS_R_UNKNOWNSYSTEM.Name
FROM
    SMS_R_UnknownSystem
WHERE
    Decommissioned = 0