I have the same issue. My car alarm randomly will honk 5 times, then stop. I believe this is the theft deterrent system malfunctioning, probably old sensors that have become too sensitive. Your manual describes how to change the settings for this system to be less sensitive, I just never bothered.
If you lock the car with the key fob, the system is engaged. If you give it a good shove or two with the system on, it'll probably do the same thing.