Both the top and bottom hatch - the lock mechanism and the bottom latch that fastens it shut - like
@scotman says, I will add that as the thing ages (vehicle in general) the weatherstrip and it's ability to rebound and seal deteriorates over time. So keeping the seal in good condition and making sure the latch doesn't bounce when it's locked. Else the weight of the rear hatch bouncing - keeps the sensor thinking it's not seated so it sets off the alarm. Adjust that bottom latch to help keep the rear hatch from shaking may fix this problem.
To find that out, simply shut the hatch, then without pressing the button, gently but firmly press down and up on that rear grasp to see if the hatch moves - even side to side - for if the hinges or gas struts are not working correctly - they can keep the hatch lifted for some time until they seat (both Hinges and the gas struts) making the hatch appear shut, but has tension upwards from the gas and position of the bad hinge to lift. So, until the struts and hinges settle into place (like the vibration and motions when you drive) it will stay that way - then, later the hatch "falls" and rests - but will not fully seat because the lock is now set, but the sensor is not - it still thinks it's "ajar".
It can be a simple adjustment onto something more major as further towards the front of the car - any impacts to the side of the front fender? There is a harness that routes from the kickpanel on both the passenger and driver sides that routes out and to the back.
Any collisions or side impacts especially to the rear - can affect the harnesses - we have had one vehicle have issues with when they backed up to the stop for delivery; the rear bumper smacked the ledge they pulled work from - causing the internal harness, close to the impact side, to get pinched. Making those harness wires bend and move in such a way they had their insulation crack and start having problems. Seems it caused shorts - from braking - the light activation made the latch buzz from power being applied from the damaged pinched wiring. A simple bump or punch from a strike can jar the harnesses - making other wires get power from other wires inside the bundle and cause issues.