I don't know on this specific model as yours has an immobiliser so probably a more modern affair but generally speaking the standard Ford "Anti-Theft system" does use the horn as a siren, they don't have motion sensors, merly just activated if the doors/boot bonnet were open. Famours with old Fords is the boot light staying on due to a faulty switch etc so I would check along with making sure all doors etc are shut.

Check the bonnet switch the rubber on the switch which the bonnet rests on as after a while this wears down and so the switch earths on the bonnet triggering the alarm.

Martin