Welcome Aboard!
One topic per issue, please - Unless the horn blows twice. ;-}
Since you are in Northern California, and presumably at some elevation, it may be relatively cold there. I had a '95 LeSabre which would do the same thing when parked outdoors in temperatures around -10°F. The vinyl horn sounder pad/trim cover over the steering wheel would shrink in the cold temperatures, and would close the contacts for the horn sounder. Of course, it seemed to always happen at 4:00 AM when it was just getting nice and cold.
My solution was to disconnect the horn relay until it was warmer. Simply disconnecting the horns themselves still allowed the relay to operate, and it would slowly drain the battery - Only when it was cold and the battery was already challenged.
I eventually removed the steering wheel pad, carefully disconnected the SIR (air bag inflator) and stretched the horn sounder vinyl cover. That only lasted about a year, when it started happening again the following winter. I finally stretched the cover so much that it distorted a bit, but it never did it after that. I owned that Buick until June of 2016 - Darned good vehicle. The horn problem began for me in about 2012, or over 15 years after it was built.