I really don't think that's a mirror. You'll notice how you can't see Rin or the dude that just did Rin. If I had to field a guess, this is Exhibitionism and Rin's in some sort of Room, like one of those recording rooms (it's clearly not one, but I'm just using that as an example), having given veiwers (And Ren) a very entertaining show.
Anyone else think so? I don't wanna change the tags unless someone else agrees :-/
I think it's basically a peep show for vocaloid fans. Len finds out about it and tries to stop her from behind the window. And lol at the guy with the phone...