We are finding that several of our Assets disappear after a time.
We go through the Add Asset process, choose a legitimate .jpg file, select 'Mark as library asset' and for a time the asset appears. We choose the default bucket. It's the only one we have.
24 hours later and many of the ones we added are gone.
We went through a clearing out process, removing all dupes and are in the process of building up our Assets library for future use. Some of the ones that disappeared have not yet made an appearance on a page, but my understanding is that 'library asset' status protects unused ones from being purged.