I think it was meant to be perhaps not necessarily noticed but at least deliberately positioned like that, and not a UI glitch. Look how it's situated directly between the microphone and speaker indicators on the top bar, and the screenshot appears in a section that says, "the indicator now shows both input and output devices right in the popover".

> I think I actually like elementary's evolution of the UI styling (from when it started as a pretty direct clone) better than I like Apple's

I'm a lifelong freedesktop user who doesn't even own a Mac but will nonetheless defend OS X's UI, from around the time of Snow Leopard or Mountain Lion or so, as the best visual design ever with respect to graphics for a conventional desktop operating system and utterly timeless—with some notable exceptions. One of those exceptions was Apple's longterm insistence on its terrible tab look—with tabs shown detached from the content its associated with and attached to the window chrome above.

It's really irking that Elementary copied that tab look, because absent any admiration, it's basically indefensible. Elementary's copying it strikes me as an example of the kind of irrationality that originates from conservatism-for-the-sake-of-it mixed with how people come to take pleasure in idiosyncrasies and other incidentals. (Similar to how when Pluto was designated not to be a planet, it led to people sort of staking their identity on choosing to insist that it be treated as one and publicly aligning themselves with "Team Pluto" for the quirkiness.) It's one of the things that they should have deliberately opted to break with the influence in order to be "better than Apple's".