I have a 2005 GMC Yukon that the rear hatch does not lock. Wouldn't be a problem but no manual key hole to lock it. Must be some kind of lock sensor. Is on the driver door panel or by the hatch?

The rear hatch locking is computer contolled .
The BCM also runs diagnostics on the liftgate lock relay control circuit and will set DTC B3825 as current and also send a Class 2 message to the DIC. The DIC will display REPLACE LIFTGATE FUSE to inform the driver that a fault has been detected in the liftgate lock circuit and that the liftgate is not locked. Faults that will set the DTC and cause the DIC to display the message could be any of the following:
• An open ECC fuse
• An open or short to ground in the liftgate lock relay battery positive voltage circuit
• A faulty liftgate lock relay
• An open or short to ground in the liftgate lock relay control circuit
• A short to ground in the liftgate lock actuator lock control circuit
• A faulty BCM

The liftgate lock relay supply and coil sides receive a constant voltage through the battery voltage supply circuit and the ECC fuse. The normally closed contact is connected to ground and the control side is connected to the body control module (BCM). A lock function will occur when a door lock switch activation, keyless entry lock transmission, auto door lock function or a liftgate lock function has been received by the BCM. The BCM will then ground the control side of the liftgate lock relay. This energizes the relay and allows voltage to be applied to the liftgate lock actuator lock control circuit. Since the other side of the liftgate lock actuator is connected to the normally closed contacts of the door unlock relay to ground, the liftgate will lock. When an unlock function occurs the BCM will ground the control side of the unlock relay and the rear doors and liftgate will unlock.
The power door lock system consists of the following components:
• The driver door module (DDM)
• The passenger door module (PDM)
• The body control module (BCM)
• The driver information center (DIC)
• The rear door lock switch
• The door lock relay
• The door unlock relay
• The liftgate lock relay--utility vehicles with liftgate only
• The reversible door lock actuators in each of the doors and liftgate
• LOCKS 20A fuse (rear lock/unlock relay supply voltage)
• ECC 10A fuse (liftgate lock relay supply voltage)

If i were to guess i would say you mite need a new lock actuator , but i don't guess . You could take the hatch inside panel off an check for B+ voltage for the lock actuation as someone pushes the switch . Or have it checked for codes .