Well, the short of it are three issues. The horn buttons stick, but that's not you're problem unless you sleepwalk (sleepdrive?). You could have a short in your horn relay. You could have a moth or bug in your vehicle that set off the motion sensors. I don't know what the D2 alarm sounds like when it goes off, but shouldn't it be a horn honking pattern, not a solid hoooooonk that never ends?
Mine happened on the silver 04 S model I used to have. The horn went off solid at 4am. I couldn't get it to shut off until I started it with the key. Evidently that blew the horn fuse and it shut off. The dealer said I had a short in the wiring somewhere, so they borrowed the relay/wiring from the ACE system (since I don't have ACE) and used it for the horn. Never happened again.
I have had the buttons stick on my G4, though. I just got it fixed at the dealer (my favorite warranty term: "renew"). I was driving and a guy was coming into my lane. I meant to tap the horn at him, but ended up as an eight second madman hornfest. Fortunately, I was able to pry it unstuck with my fingernail. Quite embarrassing.