Hi,
AltaVault does not manage the actual "images" - that is done by the host backup app (NBU in this case). AltaVault handles the translation of those NBU image files into the corresponding subset of deduplicated slab files which are stored in cache on the appliance, and the global set of deduplicated slab files which are in the cloud storage provider. In other words, only the backup app is responsible for handling the image life cycle - AltaVault simply maintains the data that corresponds to those images.
You can use the prepopulation GUI interface page on the AltaVault appliance to see the % locality of any one of your NBU backup files on cache, and correspondingly the % that is in the cloud. If you feel that there are images that AltaVault is aware of that NBU isn't, you'll be able to see the backup files here and figure it out.
Regards,
Christopher