Fixed your title.
There is definitely attached flow on the Prius' rear glass.
Your tuft test would bear this out. Please make a video if you do - it's not much effort at all.
EG: see this tuft test of the rear glass of a 1st generation Honda Insight (showing attached flow):
From: http://ecomodder.com/forum/showthrea...ght-19299.html
Perhaps the reason you're not seeing rain streaming down the rear glass is because streams on the roof can't transition to the hatchback owing to the gap (gutter) between them.