This is similar to the "Add a new Hub, Appointments, to Stratus and Spectra" but keeps it within the Calendar hub as it is just another way of viewing the calendar. It could be a list Sessions and Invoices Hubs for easy sorting and filtering, or more of a graphic like the current calendar views. Also users are used to the Agenda view within smartphone apps, for a quick filterable list of what is coming up this would be natural and intuitive.

Currently the only way to search for an unlinked appointment is through the dashboard reports / data view. This would also be more intuitive and less of a pain.