Incessant honking + red triangle of death

My 2008 Prius is honking incessantly. It's a really weak, pathetic, but very-much-there, constant horn blowing. I live in NYC with street parking, so I was parked a block from home and just happened to walk past it (definitely made friends with the neighbors over this), so I have no idea how it was going on. I was able to turn on the car (still had enough juice to start), but found a red triangle of death when I turned it on. It continued honking, even with the car turned on, so I unplugged the battery. Based on some other recommendations here, I changed the 12v battery, but unfortunately the honking continues, and now the car won't start. Has anyone experienced anything like this, and any suggestions before I tow it in to a shop?

It only has about 120,000 miles, and I'm the only owner, but outside life has been rough for this one.

 

Thanks for the recommendation on the OBD2 scanner, PriusCamper. My last one stopped being supported, and I never replaced it.

Turns out the issue was the whole fuse box deteriorated from all the weather, salt, and 13 years of outdoor parking. That was the cause of the intermittent starting and shorting the horn. My mechanic happened to have a fuse box from a scrapped 2nd gen Prius on hand, but it didn't have enough circuits to fit my 2008 Touring model. He advised we could search eBay for a used one, with potentially variable success. New OEM is $1200 before labor, but considering my milage is so low, I'm going to replace it. Fingers crossed!