Car alarm goes off after being locked. Doesn't happen all of the time. Wrong?

Yes. I had the same problem on mine in 2010. Check the passenger side hood hinge. Lift the hood, and over there you will see a little switch. It had migrated on mine and caused the issue. Make sure it is back where it belongs, secure, and the connector is clean and plugged in. Also, don't forget, it will drain your battery if it registers that the hood is open over night.