The earliest production FGxxST-14 models did not get the operator presence system, so if you don't see a hole in the hood, it is one of the few earliest ones.
Unless this truck has had some kind of modification (think...improper) to tie the OEM horn to some kind of alarm set up, like low oil or high coolant temp.....there is really no reason for the horn to react at all to the key switch.
I have seen some low oil/high temp systems that do utilize the OEM horn as an alarm by adding a grounding relay to the horn ground circuit (much the same as adding an extra horn button on the dash).