The case of the missing electrons has been cracked. Mike Spinelli and Ray Wert (both of Jalopnik fame) had rented the very same Volt that's in the previous post, drained it's battery, and dropped it off back at Hertz just before my dad and I picked it up.
Here's a picture Jalopnik took of the navigation screen displaying trip info:
And here's a picture I took, hours later:
Note the 29.9 miles travelled by battery in each, and the decrease in miles per gallon as the batteries ran out and the engine came into play.
The Jalopnik article was called "New York Isn't Ready for Electric Cars," but since Hertz didn't bother to charge the Volt before renting it again, I'm not sure they are either!