Re: Lockdowns aside
a bus commute home is a very big exception
And would make for a great big pile of false positives if OP were incorrectly diagnosed as infectious.
That's one glaring problem with contact-tracing applications. The precision of existing SARS-Cov-2 tests is poor, and given the large groups we'll need to test to make contact tracing useful and the low overall infection rate, the false-positive paradox is going to bite hard. When that's multiplied by probabilistic - and not very accurate1 - contact tracing, the number of people who will be informed that they might have been exposed is going to go through the roof.
That was part of Ross Anderson's argument; the other part is that many people will respond to a flood of false-positive warnings by calling emergency services and/or going to medical facilities for testing or treatment, which will increase strain on those systems. And many other people will see the flood of false positives and ignore the contact warnings, rendering the apps irrelevant. And others will abuse the system (to force closures at schools and other facilities, to harass, for "art", for the lulz).
Personally, I doubt contact tracing will make a significant difference in controlling COVID-19.
And those with access to the data will certainly abuse contact tracing in any way they can. If history tells us anything, it tells us that.
1Because BLE is not a very good proxy for exposure to a significant number of virons. It's barely adequate as an estimation of overall distance, and completely uncorrelated to many types of barriers (walls, PPE), surface contact, air movement, etc.