Cecelia - I agree that this would be very helpful. One way we've managed to work around this limitation is to look up the student in Core>People Finder, then on the Access tab click "Role Membership" then click View History. That will show you the date on which a student's role changed, for example from Candidate to Past Candidate.

That said, it would indeed be quite beneficial to have this be a searchable (or at least displayable) field in a candidates list.