The incubation period runs out to around 27 days, though above 2 weeks is uncommon. It seems possible one of them already had it before quarantine, and just happened to be one of those who had a long incubation period.

https://www.worldometers.info/coronavirus/coronavirus-incuba...

I obviously don’t know whether that’s the case here but its also possible that some of these anecdotal stories (I’ve heard similar from colleagues) involve someone in the same household having had an asymptomatic case acquired prior to quarantining and transmitting it to others in the same household who then get symptoms after a week or two.

I'm in the East Bay and was quarantining for almost a month before my symptoms showed. I also assume I got it at the grocery store since that was my only outing.

Are they not taking public transit? Your friends must be some of the most unlucky people ever. The Bay Area has had under 3k cases since the start of April outside group living settings (nursing homes, homeless shelters, etc.). That's ~1/2000 chance of getting covid from anywhere outside.

Remember that’s not a count of cases. That’s a count of the under 3000 people that came up positive after permitted to take a limited availability test.

Take total case deaths, divide by CFR, that’s how many likely total infected there were 3 weeks ago, and then compound daily by the daily spread rate to find how many likely today.