Horn goes off when unlocking
 

So I'm not sure why this happens sometimes and not others, but when I leave my car to go somewhere and turn the key to lock the door (my fob doesn't work) and arrive back, when I unlock the car it will honk. Same type of honk if you lock the door with one of the doors open. When this occurs, I know that when I locked it that the honk didnt go off. I looked in manual and can't figure it out. Why does this happen sometimes and not others?