We have a calendar that is not embedded in any page, and users that are not watching the calendar, or the space in which it resides receive notifications for new events and event edits.

Checking the user's list of watching items does not include the calendar for which they receive notifications, or the space where the calendar resides. Using a query to see if the calendar is embedded anywhere returns no results:

embeddedSubCalendarId:CALENDARID

While the same search using a different calendar ID does show results for pages where the calendar is embedded.