car alarm sounding after opening drivers door

What is your question?

Hi all... I drive a 2018 Ford Focus Sel, 27.8 k miles on it. Today, around 11 am I stopped to drop off some mail, ignition off, opened door. Immediately there was a series of beeps like a timer, then the car alarm went off! I pushed the red button on the key fob repeatably but there was NO effect! The alarm stopped when I pushed the button that unlocks the door? Not sure what to make of this? Was I able to open the door when it was locked, thus setting off the alarm? I live in Iowa, today it was cold! Was the alarm affected by the cold? I had been driving the car for about 20 minutes before the alarm sounded, and it was nice and warm in the car. I've had the car for about 10 months now, this is the first time the alarm ever went off.. What to make of this? Nonplussed... Thanks in advance for any suggestions....

How long has your 2018 Ford Focus had this problem?

First time it ever happened was today