What is RebootRequiredCheck checking?

My installer is shelling out install SQL Express 2012.  Before it does so it is checking this Registry Value:

Key: System\CurrentControlSet\Control\Session Manager  Value: PendingFileRenameOperations

However, even when it is empty the SQL Express installer is sometimes failing the RebootRequiredCheck.

Anyone know what else the SQL Express installer is checking for the RebootRequiredCheck?

Thanks.