I dont think the alarm was activated by the passenger door opening.. that was just timing. The alarm gets activated by the remote. If there is entry to the vehicle without using the remote such as windows down and pulling up the knob or not doing the Keylock sequence to deactivate.
The alarm sounds once someone attempts to start the vehicle. I presume key or no key (hotwire)

Most likely what happened was you locked the car with the remote and possible someone else from your household made entry to the vehicle without the remote. Once you went back thats were you had your problems.