GMC 2010 Sierra - Manual (Page 151)

Lockout Protection
This feature protects you from locking the key in the
vehicle when the key is in the ignition and a front door
is open.
If the driver's side power door lock switch is pressed
when the driver's door is open and the key is in the
ignition, all of the doors will lock and then the driver's
door will unlock.
If the passenger's side power door lock switch is
pressed when the front passenger's door is open and
the key is in the ignition, all of the doors will lock and
then the front passenger's door will unlock.
Rear Doors
Extended Cab
Your vehicle may have a rear access door(s) that allows
easier access to the rear area of the extended cab.
To open a rear access
door from the outside, first
open the front door. Then,
use the handle located on
the front edge of the rear
access door to open it.
You must fully close a rear access door before you can
close the front door.
3-13