When does SnapCenter delete old snapshots?

[mellow lodge]

I have been looking into how and when SnapCenter actually removes old "out of retention" snapshots from the storage system.
I guess that the SQL Host does this task because it also creates the snapshots, and I can confirm this in the SMCore logs where I can see the "SnapshotManagementServiceProvider::CreateSnapshot" and "SnapshotManagementServiceProvider::GetSnapshot" and "SnapshotManagementServiceProvider::SetSnapshotLabel"... but there are no "SnapshotManagementServiceProvider::DeleteSnapshot" or similar... actually the one ones I can find in the logs are the three first ones (Create, Get and SetLabel...) So my question is.. where and who does the deletion of the snapshots?
The reason I am asking is that because of a configuration issue we ended up with a log of "orphan" snapshots on our storage systems "900+ snapshots" per volume... and we would like to make sure that the snapshots are deleted acording to our policies.
We would also like to clean up all these snapshots, so if anyone have a script that can check volumes snapshots up against snapcenter snapshots, and provide a list of orphans, and maybe also delete them, that would be very useful... of cause NetApp does not provide such tool... What I don't understand is why SnapCenter tries to delete expired snapshots.. clearly fails at this task... then continues to delete its records from its database... ??