Horn honks intermittently

I am somewhat new to this forum but I didn't see this problem posted before. I have a 2008 Prius in top shape, low mileage (only 70,000), but has in the last year developed an intermittent honking problem. I have a video I took while this was going on, if I can post it I will. This started when we were in Madras, OR last summer for the eclipse. While driving back in slow traffic and in hot conditions the horn started to beep intermittently - short gentle beeps from 3 to 7 or so in a row. Folks would stare since it sounds like we are signaling impatience or something else. The problem settled down for a while, then resurfaced recently after we had stored the car for a few months and asked someone to drive it occasionally to keep the battery charged. The battery ran down about a month before we returned, and while charging up the car there were several episodes of loud horn honking, notably when the key was in the ignition and the door closed. Then after the car was charged up and after we had the battery checked out at the Toyota dealer, the car started the intermittent beeping again, every couple miles or so, while driving. It is very annoying to us and other drivers in traffic. Again we went to the dealer, but they could not figure anything to do, except they found "alarm was turned off in custom settings, turned alarm back on". This actually doesn't make sense that this would stop an alarm, but for a few days the horn stopped honking. Now however it is back with us full force. Very annoying and frustrating. We have also heard the horn beep like this when turning the car on. Appreciate any ideas on this.

 

I don't have the reading, but I did have the 12 volt battery in the back checked at a garage, there was no problem.