Factory alarm?
Factory alarm? 
Your shock sensor is at fault here most likely.
Pull your panel in the hatch where your spare tire is. Down on the wheel well you'll see the little black shock sensor with a red LED on it. The LED should be off. Watch it for a bit to be sure it is staying off. When you tap it, the LED should come on for a couple seconds or so then turn back off.
If it doesn't do that, the sensor is bad. Simply unplugging the sensor will keep it from acting up on you though. It won't set off the alarm if someone jostles your car around but at least it won't go off randomly.
Also, if it does this after the car is wet but fine when dry you may have a leak and water is getting on the sensor. Mine gets damp from hatch runoff sometimes, which has made it go bonkers temporarily until it dries out.
Pull your panel in the hatch where your spare tire is. Down on the wheel well you'll see the little black shock sensor with a red LED on it. The LED should be off. Watch it for a bit to be sure it is staying off. When you tap it, the LED should come on for a couple seconds or so then turn back off.
If it doesn't do that, the sensor is bad. Simply unplugging the sensor will keep it from acting up on you though. It won't set off the alarm if someone jostles your car around but at least it won't go off randomly.
Also, if it does this after the car is wet but fine when dry you may have a leak and water is getting on the sensor. Mine gets damp from hatch runoff sometimes, which has made it go bonkers temporarily until it dries out.
