What I've learned is that "lockdown" is a broad term, not a binary one. Quoting https://en.wikipedia.org/wiki/COVID-19_lockdowns :

> Due to the COVID-19 pandemic, a number of non-pharmaceutical interventions colloquially known as lockdowns (encompassing stay-at-home orders, curfews, quarantines, cordons sanitaires and similar societal restrictions) have been implemented in numerous countries and territories around the world.

For example, by "lockdown", do you include things like limiting venues to 50% occupancy?

Let's say "a single case is detected" .. in someone who just entered the country and is in quarantine, and the quarantine protocols are effective.

I would do nothing different.

Let's say that single case is detected in someone who just broke the world's record for most number of kisses in 8 hours - 11,030 says https://www.guinnessworldrecords.com/world-records/68045-mos... .

Then my reaction might be a bit different.

Since the circumstances are so varied, I don't see how your question can have a reasonable answer in an HN thread.

Except for Broken_Hippo's entirely reasonable answer.