Based on what I found on the Internet and info shared by my supervisor(an immigration lawyer), I would like to share the following:
1. Tracker update now only refers to system automatic refreshes(according to the Twitter response of IRCC). Two possible outcomes: A. there is something changed, B. there is nothing changed. In both circumstances, however, your application process is not accurately reflected.
2. You may see an update like "We sent you a message..." on Tracker. This message could either be a 'real message' like a medical request, AOR2 etc., or it also could be a simple glitch: there's an upcoming email in the system(hasn't been sent yet), or there's nothing(according to various phone call info collected from my supervisor's clients and on the Internet).
3. Ghost updates are only traceable on the GCKey account now: the status of your application changed from 'Submitted' to 'profile updated.' And there will be a notice sent to your email.
4. No matter the application type, the Tracker will have details whenever there is a decision(Decision Made, DM). Like CoPR details, Study Permit details, and even Visitor visa details(according to my supervisor's clients' Tracker accounts and previous info shared in this thread).
5. In the no.4 situation, you may see both GCKey and Tracker showing 'processing', 'in progress', 'not started' or whatever. You may have an email beforehand, or you may not.
6. The most accurate way to track our applications is to request IRCC/CBSA/CSIS notes.
7. 'Not started' on Tracker does not necessarily mean 'not started yet'. It could be 'started or even done'.
8. 'We are processing...' on GCKey may refer to the BG of PA has already been checked and completed or assigned for a further investigation conducted by CSIS/CBSA.
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Luck is the determiner in many applications. I see inland application sail through within 30 days and outland application within 4 mo.