I suspect that the throttle switch
(Lighting switch on internal controllers) has been utilised as a Reverse switch on the external controllers and is therefore connected to the Reverse connector on the controller.
The horn beeping is probably the Reverse "Sound" warning function which can be disabled using the USB programming lead and software:
Check if the bike go backwards when you apply the throttle with the throttle button pressed, as this will hopefully confirm my suspicions.
It might be advisable to unplug the horn first to protect your ear drums.
If the two wires coming from the throttle switch are completely isolated from everything else inside the throttle
(which they should be) you can simply use them to switch the +5V supply to the Pedelec sensor by cutting the Red PAS wire and connecting the two wires from the switch to the two cut ends of the Red PAS wire.
Alan