In like fashion, when you lock the vehicle there is usually a horn indication to tell you that the alarm has been activated/set.
Honk is the cry of a wild goose
You probably have a short in your electrical system. Also the fuse may be burnt out on the horn.
Sounds like this car has the remnants of an aftermarket alarm or keyless entry. If the alarm has passive arming turned on and is malfunctioning, it can cycle the doorlocks and or honk the horn or siren (depends how it was installed. you could also get your wiring and fuses checked.
What can honk without using a. Horn
Honk
A goose.
pulling a string
The problem is in the alarm. If it is GM take it to the dealer.
Europeans...
toot
A goose
push the center of the steering wheel. honk honk