Maybe draw a lighter square around the button in the sidebar would be
enough?

Maybe something like this is clearer (and less ugly ;):

If you don't have the currently playing something focused, the
RBPlayer widget reads "Not Playing". This has been bugging me for a
while since it's playing, since there's sound coming from the
speakers. It's only that source that is not playing.

So how about replacing "Not Playing" by something like this:

"Playing from the Library" or "Playing from the "House" Group"

Ok it solves 2 problems at the same time :)

Something bothers me still, what happens if you play from the library,
then go to a group that contains the song being played? Does the row
shows it's playing, what about the RBPlayer widget...

nope, it doesn't. and imho we shouldnt do something like that, will be
quite confusing i think ..

done