"Site Updated" banner

Today on meta I saw a new “Site Updated, please refresh…” banner. (I can’t post a screenshot because it’s gone now.) However after updating my own site, I do not get the banner. Can someone please explain what triggers the banner? (Updates, obviously, but why didn’t I get it on my site?) And can it be triggered by theme updates? Those are likely to need a refresh in my experience.

It is a change that we recently merged into core. If you hang fire for the release in a couple of weeks you’ll have it.