I had something like this happen to me after I had the car stored one summer. When to start the car and then wondered why it looked like the headlights were on. Then i realized they weren't. The wire from the battery to the horn relay was on fire. The car was off, w/o the ignition on, but the wire kept burning.and shorted the battery. Hydrogen was flaring off the battery. I freaked out and just ripped the battery cable off. Turns out it was a bad horn relay. I got a new relay, a new fire extinguisher, and some new underoos.

Hopefully it's something simple.