I have a 1992 Foxbody Convertible. First one and just bought it used. Previous owner disconnected the horn because every time he unlocked his Chevy Truck with key fob, Mustang horn would sound as if someone hit panic button on a ford key fob. Beep, Beep, Beep about 6-8 times. It happens with my wife's Chevy Traverse key fob too, she unlocks car with fob, Mustang panic mode kicks in. I also noticed this when I went to a particular gas station too. Shut motor down, removed keys, got out...

Horn's Panic Mode activated