[Keith Z in DC]

I ran into an issue with my factory alarm last night. I attempted to lock the doors/arm the alarm with the button on the key and the turn signal indicators activated twice and a few seconds later the horn activated. I have had this happen in the past when a door was not shut all the way. I checked all the doors, engine compartment, hood, sunroof...everything was closed. Is this normal? If so, the horn did not activate for the first month when I armed the alarm (the turn signal indicators have always flashed twice when armed). Can anyone shed any light on this for me?

Thanks,
Keith Z