With the permission of Michael Waldman, I’m publishing here the email that he shared internally today.
There was a recent change in how Aleph handles records that have an Item Status of “Missing” and “Lost”. In the past, in the Catalog, the record was displayed as “Lost”. This was a status that Circulation would add after extensive searching. However at this time many items that were “Missing” (i.e., not fully searched) have been changed to “Lost”.
At this time however, these records are showing “Please check shelf”. I encourage you to send me any such record that you encounter for further investigation, but it is likely that the book is not available (or immediately available) and the patron should be directed to another resource.
Please let me know if you have any questions.