My brother in law flew from San Francisco to Toronto for Christmas. Did his quarantine at an Airbnb. He's heavily into tech (MSc from MIT in computer science) and took a long look at the app. He said that the quarantine app tracked his location through the phone (he didn't leave the condo for 15 days). No knocks but did get phonecalls. Everyone else I know has had knocks as far as I know.
I suspect that if it does track your phone and you were shown to have movement you would get a knock...
Was he required to download the app? If so, is he a Canadian citizen? Also, is it the Covid-19 app many have downloaded or a quarantine specific one? Haven’t heard or anyone else having to download any app to quarantine.