Hi Robin, notwithstanding @Chris_Hofer 's advice, I would recommend talking to someone who has access to the User Account Details function which lists forensically every clickview and action. This includes every URL address and whether the action was a 'GET' or 'POST'.
Nothing really disappears from Canvas, but it is possible that the assignments were deleted - in which case the 'undelete' function (added after the end of the course ID would list these.
I always think its an omission that page history is not available for Syllabus and Assignments, but they may be findable through an API call.
Not sure if that helps.