As well as training pilots, I train flight followers, the people responsible for, in the words of the legislation ...
CARS 722.12 (2) Flight Following
Flight Following for a Type D system is the monitoring of a flight's progress and the notification of appropriate air operator and search and rescue authorities if the flight is overdue or missing.
Naturally if I train people to notify the authorities when a flight is missing, they have to know what that means, so I looked it up. First hit was on Wikipedia ...
According to Annex 13 of the International Civil Aviation Organization, an aircraft is considered to be missing "when the official search has been terminated and the wreckage has not been located"
That doesn't make any sense, but it wouldn't be the first time Wikipedia was wrong or that Canada had a different definition than the international one. I find a Transport Canada source.
Advisory Circular (AC) No. 100-001 Subject: Glossary for Pilots and Air Traffic Services Personnel
An aircraft is considered to be missing when the official search has been terminated and the wreckage has not been located.
So, according to Transport Canada, I am to train the flight followers to report an overdue aircraft promptly to SAR, and then, weeks later when the search and rescue experts have failed to locate the aircraft and advise that they are discontinuing the search, to report the missing aircraft once again to SAR. At least I won't have to look very hard for relevant humour to spice up this part of the instruction.