Render midi, using bounce to tracks, for a group of solo'd and selected tracks.
 
Unsolo those tracks.
 
Solo the resulting audio track(s) from the bounce.
 
Open console view in order to view the original tracks and the solo'd audio tracks.
 
Play.
 
Oddly, troublingly, the non-solo'd, original tracks, show audio activity.   Seems as though it shouldn't be possible.
 
I believe the audio activity in the non-solo'd original tracks only occurs on First play -- repeated plays, no audio activity.
 

Audio or MIDI meter activity? I've reported previously that MIDI tracks can continue to show activity when solo is initially enabled on other tracks, and the transport has to be restarted to correct it. But I don't think I've seen it with audio. It's a display issue only, in any case.

I believe midi only, in what I've observed.  But I remind you of the 'bounce echo' audio that is plopped on the beginning of a new and unrelated bounce. 
 
It might be worth investigating:  if a midi track which should not be sending any data DOES send to a VSTi (synth) -- what happens?
 

Let's not muddy up another thread. FX tails and buffering are a whole different animal. And in this case I'm quite sure that MIDI never gets processed. It's just that the meter activity isn't initially suppressed as it should be.
 
Also, just to be clear, it has nothing to do with bouncing per se. If you solo any other track, including a recorded audio track, the MIDI meters in unrelated tracks can continue to show activity.
 
EDIT: Also, note the opposite phenomenon occurs as well. When you unsolo a track during playback, MIDI meters on other tracks remain inactive until the tranport is restarted though you can hear them playing.