My car is honking non stop! But why?

I have a 2010 Prius with 175,000 but today I went to order some drinks inside Starbucks and while in the line I hear a car honking non stop. I left my car with the engine on but when I went outside.. my car was honking non stop. I had to unplug the battery and unplug the horn. Its not the alarm, I even try pressing the horn, I check the locks and everything. I had my friend looking at my car and he said it might be a Relay. So.. my question is... what is making my car honk non stop? I need some help.