I can't help with this but am hoping someone else can because I have the same problem. My 99 Accord did this the other day. I locked the doors, after about 1 minute the horn started honking. Of course, it's random (only done it that one time, stoppped afer I started the car) so I don't expect a garage to troubleshoot it.

I noticed a few nights prior that alarm would not arm, like when the hood is up, for example. I started the car, shut it off, and I got it to arm... have to think the two are connected....