Hey MKguy, it was indeed your option 1, the RDM's
a simple cli command: Get -VM | Get-Harddisk -Disktype "RawPhysical", "RawVirtual" | Select Parent,Name,DiskType,ScsicannonicalName,DeviceName,CapacityGB
gave the numbers for all RDM's and this was the difference i saw between the datastores and all vm's used space :-)
Thanks a lot for your help, much appreciated!