It is wonderful that we can now place a Hold, pick up materials when ready, and then return them to the book drop of almost all San Diego Libraries. However, you might have gotten a notice during the process that an item has gone overdue. Perhaps you had already returned it.
What is going on ?
After items are returned, they are not touched for four days. Then they are checked in. This process is based on guidance from the "REALM Project" a collaborative project to learn how long COVID-19 is present on various library materials. OCLC, the Institute of Museum and Library Services, and Battelle are conducting research on how long the COVID-19 virus survives on materials that are prevalent in libraries, archives, and museums. The project will draw upon the research to produce authoritative, science-based information on how—or if— materials can be handled to mitigate exposure to staff and visitors.
The current consensus is that after four days, the materials can be safely retuned to the collections for reissue.