So I turn power and start coasting down from the 3rd floor parking to the lower level. Foot is not on the accelerator as gravity has taken over. Shouldn't my MPG be pretty high but instead I see 20 - 30 MPG all the way to the lower floor..
No because the engine is running. You have to go through the warm up sequence before you can coast/glide with the engine off. If you use the EV button you can avoid this warm up penalty until you are out of the parking structures summing you stay under 10mph.
You also can not have the heat on or a temp set above "low" or the engine may start. If the battery is low it will bump you from EV too.
Also, if you had drained the battery before parking, the car may be working on replacing the lost battery charge, especially after the first stage of warm-up has finished (if it takes more than a minute to get out of the garage).
