I think I found the issue. I tested it several times and it worked. There is a small metallic peace in the car key (you can see two small pieces on the 2nd part from the left). I found the one in the center detached. I have reposition it and glued it back. I have to do it every 6 months as it become loose again with time and with the exact same result : horn going off randomly 24/7.
