A couple possibilities I know of:
1. Did you have your key in your pocket and something else in your pocket just happened to hit the alarm button?
2. You do have motion sensors, they are located on the B-pillars, there should be a button at the base of the b-pillar that can be used to shut it off if you need to trouble shoot the problem. The sensor could be going bad, but this is pretty rare.
3. Most likely: The switch or wiring on your door lock assembly is failing. The car thinks the door is open even when closed and since the vehicle is locked, the alam sounds. This is most commonly the drivers door since it is used the most. If your LCD isn't completely pixilated, it would show the door open icon even when the door is shut. Another symptom of this is that the dome light comes on and off while driving. The car will also beep at you frequently since it thinks you are driving with the door open. Swap door lock assemblies with someone to diagnose and check the rubber boot between the door and the car near the A-pillar, this is where the wires fray and fail. You will have to remove the window frame to access the door lock assembly, there are a couple adjusting shims that you should mark before removing the window frame.