I own a 1997 Oldsmobile 88LS. With the doors unlocked and the key in the ignition I opened the drivers side door to get in and the horn starts to honk for 15 to 30 seconds and goes off by itself. I have had the doors locked and unlocked the driver side door only and opened the door and the horn has done the same thing. On both occasions I have put the key in the ignition and turned on the car to see if the horn stops and it doesnt. I have turned on the car and drove away the horn is still honking and goes off in about 15 to 30 seconds. This has happened about a handful of times at random times in the morning and night.
Your only recourse, in the end, may be to have an automobile security and alarm specialist neutralize that turkey, er, I mean defective security system.
does this car have a “viper” or other after market alarm installed? (even if you dont use it, is there one?)
Try replacing the battery in your key fob. If the battery is low it may be intermittently giving the security system a wrong signal.