ok, so ThatAdamGuy had a thread (click here) wondering if it would be cool if the Prius gave us feedback. Well, I've found someone who did just that! View video here! There's the solution to all of us who wondered if EV mode was ever activated lol.
Ah, wow! I'm just (re)discovering this now. Thanks for the fun and thoughtful ping, and apologies for the delay in replying! and d'oh, apparently I must have set up two accounts back in the day <sigh>.