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Horn pad / air bag assembly is likely deteriorating inside allowing the ground circuit to the horn to close without being pushed - more pronounced in hot temperatures.
Just a guess but I’ve had that problem with other vehicles - kinda odd on a 2015 year model. Hands-on testing will be required.
Found problem Small nylon (pencil lead size-1/16' long holding a return spring mechanism broken ) this will cause the contacts to stay closed.