Here's what the battery usage looked like before DriveMode:
Here's what the battery usage looked like after DriveMode:
Notice the battery usage by "Device Health Application" and the GPS activity.
So what is this "Device Health Application?"
This CrackBerry forum post answered that - Carrier IQ.
From my experience it seems that DriveMode triggered something in Carrier IQ that caused the GPS to run almost constantly.
A factory reset helped that but it took the Marshmallow upgrade to completely resolve this.
Here's what the battery usage looked like after Marshmallow:
Battery usage has been fine ever since and I haven't tried DriveMode again!