Steps to reproduce
Follow Browse > Collections > New Collection
Fill in required information, making sure to check "This collection is unrevealed"
Follow Post to Collection
Fill in required information
Press Post Without Preview
Follow Search > Bookmarks
Fill in "Any field" with something that matches the work you posted and bookmarked, e.g. General Audiences if you rated the work General Audiences
This is "Any field on work" if you're on Elasticsearch 6.0
To make this easier, fill in Bookmarker with the name of the account you bookmarked the work with
Also to make this easier, choose "Date Bookmarked" for "Sort by"
Press Search bookmarks
The bookmark shows up in the search results, thus telling the world that the bookmarked item is rated General Audiences (or contains that phrase somewhere in the searchable info).
What should happen
The bookmark should not appear in the search results. It might be okay for it to appear if the bookmarker name, tags, or notes match the search term, but it should definitely not appear if the match is on hidden information.
This also happens with bookmark listings for users or tags, fyi.