I'd be willing to bet it's for an old car alarm installed by a previous owner sometime in the car's life.
I agree. I found an alarm keyed switch on a 66 Shelby mounted inside the right hand horn. It's still there (the owner is "involved in other activities") but not hooked up. Access is thru the grill. It's a tight squeeze but it can be accessed.
On a 66 Hardtop I bought many years ago, the alarm keyed switch was mounted on the outside of the left front fender, easily accessed for alarm activation and deactivation. The only problem with its location was the back side of the switch, where the two wired terminals are located, were also easily accessed, like with a pair of diagonal cutters. The switch was not hooked up. There was a plunger switch mounted on the radiator support. It's wiring was suspended in the underhood harness to the former location of the alarm horn that used one of the washer bag screws.
My current security techniques include cars parked in a locked garage.