I had my 2010 HHR alarm system going off all night long every 5 to 10 minutes after disarming it. The dome light kept coming on. After looking over several parts of the manual. I figured it out. The hatch was not shut all the way. It needed to be slammed down harder. Why was it made to have stronger people to close it? So before disarming your alarm check all doors and the hatch to be closed.

 

Welcome, sometimes the latch gets sticky, try a bit of a spray lube on the piece in the hatch, or liftgate

 

Shouldn't have to "slam" it to shut it completely. A good firm push is all that required if everything is in working order.
Check the gasket and latch assembly for any binding. Also, cold weather can cause the hatch struts to make it hard to close. Those could need replacement.

FYI. You didn't disarm it if the alarm was still going off.

 

See those black rubber blocks on the sides of the hatch opening? Those adjust the latch.