Coming from the Chevy Volt, it does something called Engine Maintenance Mode (EMM) periodically. If the ICE hasn't been run in a month or so it'll run the ICE for a few minutes to lubricate and circulate the fluids of the ICE and its systems. Does the Prime have a similar mode baked into the cars schedule?
I believe the PiP had such, but if Toyota continued it with the Primes, they made it invisible. In that there isn't a message alerting the driver for that being the reason for the engine spinning, or one of the numerous another reasons it could be running for. The Prius Prime isn't as focused on staying in EV like the Volt.
