As clear as mud
How the “LeaveHomeSafe” app actually achieves its aim is unclear. You can scan the QR codes at venues, and enter taxi license numbers, and confirm when you leave a venue. The Government has, multiple times, assured people that their visits are ONLY stored on their phone and automatically deleted after 3(?) weeks, so there are no privacy concerns. But, if you are tested as COVID +, you have the option (otherwise, what's the point?) for uploading your records to the Centre for Health Protection. Presumably, the consolidated records are then transmitted to all app users. This might cause two issues:
i) If there are lots of cases, does this scale well? Three weeks of visit date for every case must be transmitted to every app user for matching.
ii) If there are very few cases (yesterday there were 14 new cases in HK), privacy is compromised. In the extreme case, the movements of one person are transmitted to every app user. Anyone who can work out who the case was ("Bob went into quarantine yesterday") can check where they have been (depending on the in-app security... probably reasonably breakable).
In practical terms, the actual scanning of the QR is fast enough, it's the other stuff that is inconvenient. I've experienced all of these:
a) I didn't see the code on entering
b) I did see the code, get the phone out, start the app...
c) Someone (a security guard) is standing in front of the code
d) The lift arrived, catch the lift or scan the code?
e) I'm leaving... Oh, look, there's the code, scan it now?
f) I left, and forgot to tell the app (it defaults to 4 hours before expiry)
The resulting records are likely to be incomplete and inaccurate.
The size of the venues is highly varied... restaurants are now required to display the code, and eat-in customers must either scan it or record their details manually, so every corner cafe has a code. So do 20+ floor Government buildings with thousands of people. There's a large public hospital with multiple buildings with QR codes at each entrance, and some of the buildings are linked by bridges, without QR codes. Is it counted as one venue or many? Either you end up testing far more people than have possibly been within shouting distance of a case, or you miss close contacts, or, most likely, both.
Interestingly, during public briefings on the latest figures, Government officials have not reported the number of cases that were detected because of this “LeaveHomeSafe” app. I won't speculate why.