During the Falcon 9 second stage firing of the Merlin 1D Vac engine, the video shows an exhaust plume ejecting from near the top of the nozzle bell, and canted outward (say 10 degrees) from the bell. I was thinking that was the Turbo Exhaust, but based on this article, I see that that is not the case. What in the world is that exhaust plume?
The Merlin 1D VACUUM uses its turbopump exhaust as engine nozzle film coolant, it it recirculated. Discussion of Merlin 1D Vac here
If you are referring to the gaseous exhaust from the small pipe at the top of the bell/below the engine. Seen here at mission time 43:54 , that is just venting the excess pressure in the oxidizer, following engine cooldown for planned firing and continuing after the last burn. You can see it active both before and after the final burn. The spin-up for deployment occurs only seconds before deployment, and is done using N2 thrusters.