As all reservations are created in advance, MuseumKey & LendingKey cannot account for late returns. Under Library >> Settings, you can enable a feature called No. of Days to Block after Due Date.
This setting will allow you to set a number of days to block after a reservation due date to create a 'buffer'. (i.e. If a reservation due date is December 15, the pass will usually be available on December 16. If this value is set to ' 1', then December 16 will also display as unavailable on the calendar.