Jonathan Green noticed that Bibliotheca books suddenly disappeared from his collection. I discovered that if a book has no active loans or holds, the /circulation/items API endpoint will send this document:

<?xml version="1.0" ?>
<ArrayOfItemCirculation xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"/>

This document is meant to convey that there are no active loans or holds for the book. Unfortunately, we use this API to check whether there was any circulation activity we didn't hear about, and we interpret this document as meaning that the book has been removed from the collection.

We should stop using the /circulation/items API endpoint – it doesn't do what we need. The /items endpoint is a little redundant because it also includes metadata, but it always has the up-to-date circulation information we need, so we should use it instead.