My truck locks itself -

[dan42]

It wasn't running - I pulled the keys out, put them on the center console, stepped out of the truck and closed the door, and on both occasions, not only did it lock the doors but it also chirped to let me know that the alarm was set. It was about 3 seconds after I got out of the truck. I made a door key now, and hid it on the truck (the ignition keys are chipped and too expensive to make another). I'm wondering if I am hitting a setting or tripping something to happen, but I don't think this is normal.