Same thing happend on my 2004 Sienna a couple of months ago. Woke up at 3am hearing some ahole blowing his horn constantly. After about 10 minutes of this I went outside and discovered I was the ahole. I opened the hood and got a strong acrid smell of something badly overheating. I had to back off for a minute to let the smell vent as it was unbreathable! I started to disconnect the battery, at 3am it takes a bit to find the right tools. I was 30 seconds away from having the battery disconnected when it just died. I estimate the horn was blowing constantly for about 15 to 20 minutes before it died.

Two climate control systems, one inside and the other at the tailpipe.

2007 Camry Hybrid (in service June 2006)
FE: 6.77L/100km (34.8 mpg) over 37750km (23450mi)
75% city - Ave trip 20 min - 2 Winters