The horn went off when jeep was parked and ignition off

double posted. wow.
when it beeped, did you look in the display (RPM)
it tells you why?? the VFD or EVIC tells you why.? (rpm dial)
do you park with the alarm on , on the street or a garage?????
if in a garage park and turn alarm off. it does turn off the alarm.
did you bump the panic button?
eg its under your pillow?? LOL or the cat chewed it?
read your other post, it's got lots of answer there.
to find cause.
if the horns stays off, if alarm is off then its not the horn button.

as many will say. there are 4 things that run the horn.
button horn
alarm
door locks.
panic.(part of alarm but not totally)
on my car, i have most themn turned off, like the lamp flashings.
and the horn chirp is off on all my cars, all i get is the door lock click sounds, nice (neighbor friendly)
so turn all all that and see if the horn still fails.

that panic button can fail,
if mind did id pull the fob battery and then use FOB 2.
to see of the FOB fails.

your other post said driving
this one says, parked. 3am clue.

Sounds like a relay stuck on that killed the battery. I keep my parking lights on so I can see when the unit starts (no other feedback). Problem comes in when it starts and I don't know it, Parking lights come on and won't shut off till door is opened.
I think this is what killed my battery. I need to return to the installer and and have a relay installed so lights shout off when engine is off.
Hope this helps!!

The 2 wires going to the horn are switched.

does the horn sound when you press the horn switch. if it does not sound replace horn fuse, Or else check brake light switch

The security system thinks that something is wrong and is sounding the horn and flashing the lights. This is because the system had no battery power. Unlock all the doors, put key in ignition turn all way except to start the engine. remove key from ignition and lock all doors, This may reset the security system.

What year is the vehicle ?

The shift lock solenoid is not getting power due to a blown brake light fuse or the solenoid itself has gone bad.

To Larry Crowley,
Did you ever correct the problem to your 2000 Jeep Grand Cherokee and did you in fact determine that the problem was in fact the Body Control Module (BCM)?
I too have a 2000 Jeep Grand Cherokee Ltd with 162,000 miles. Up until just a few months ago, I never had any major problems with the car however in early December one evening the car was turned off and parked in my garage and the horn just went off out of nowhere, a solid horn...it was deafening! We unplugged the battery for the night so it wouldn't drain the battery. The next day I took the car to the mechanic who replaced the horn switch and relay. We thought that had fixed the problem. An hour after I left the mechanic with the car both the BRAKE and ABS lights on my dash went on and my speedometer stopped working (very scary to drive that way). When I stopped the car and went to lock it with my keyfob, the solid horn went off again so obviously the problem was not the horn switch and relay!?!? Over the past few months I have had my car to three different mechanics as well as the Jeep dealer and they have all concluded that the problem must be the BCM but they said that the only was they can properly diagnose the car is if it is acting up with the above mentioned symptoms while in the shop. Unfortunately it only acts up intermitently, not all the time. I am at such a loss. Currently we have removed the fuse for the horn so I am driving with no horn which is extremely unsafe! I look forward to any advice you have in this regard. Thank you!

Check the fuses,and then look at the fusible links,most likely on the starter,if they are good,replace the ignition switch.

Replace horn relay under driver's side dash