03 Jetta TDI randomly locks?

I just bought this car a few months ago and I noticed that it will randomly lock (and activate the alarm). The car will lock when I am sitting in it with the keys sitting on the passenger side seat and sometimes when I have them in the ignition. It will also lock when I am standing next to it holding the keys.. Pretty much whenever it wants to I think. I also noticed when I close the trunk 2 or 3 seconds after the car will SOMETIMES lock but all of this does NOT happen every time, hence it is random.. Also about once a week or so when I turn my key on the alarm will go off for a split second then end. This usually scares the **** outta me by the way and causes unwanted attention. PLEASE HELP ME!!!! I only have one key for the car and I’m afraid it will get locked in!