Hi, again! Just another idea I had:

Enemy sensor: Car will sense and target nearby enemies within range of its guns. Does nothing if vehicle is unarmed.

I've already had an unarmed truck (without enemy sensor) driving by an enemy nest and provoking biters to chase it. How about repathing instead to avoid enemies if the car isn't armed? By the way, what does "unarmed" mean? A truck without a weapon clearly isn't armed -- but what about a tank that has run out of ammo? Would that go on a suicide mission if it had an enemy sensor?

Unfortunately there's no way to tell the Factorio path finder to avoid enemy bases, so repathing wouldn't do any good as we'd just get the same cached path back.

Unarmed means the car has no gun(s). The enemy sensor doesn't check ammo, it only detects enemies and fires the gun if available. It won't send a car after enemies, nor attract enemies any more than a car without sensors does.

I suppose we could make the enemy sensor behave as a panic-and-run-away sensor when a car is unarmed...

Unfortunately there's no way to tell the Factorio path finder to avoid enemy bases, so repathing wouldn't do any good as we'd just get the same cached path back.
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I suppose we could make the enemy sensor behave as a panic-and-run-away sensor when a car is unarmed...

That is exactly what I meant with "repathing"! :-)

Unarmed means the car has no gun(s). The enemy sensor doesn't check ammo, it only detects enemies and fires the gun if available. It won't send a car after enemies, nor attract enemies any more than a car without sensors does.

According to the info page, you shouldn't need the ammo sensor either to detect whether the vehicle weapon has ammo:

Ammo sensor: Car will reload ammunition from its inventory.

I'd interpret this in the way that the ammo sensor (as well as the enemy sensor) would somehow know the ammo inventory, but the ammo sensor would check the trunk inventory for ammo if the weapon has run out of ammo, and reload it. If the vehicle has a weapon, if there is ammo in the trunk, and if an ammo sensor is installed in the vehicle, the weapon will always be loaded -- otherwise, the vehicle should go into panic mode if enemies were near.

How about this (using code from your control.lua in the following)?

908         local enemy = nil
909         local gun = nil
910         local driver = car.get_driver()
911         local passenger = car.get_passenger()
912         local trunk = car.get_inventory(defines.inventory.car_trunk)
913         local ftank = car.get_fuel_inventory()
914         local clip = car.get_inventory(defines.inventory.car_ammo)
915 
916         if car.prototype.guns then
917                 for _, g in pairs(car.prototype.guns) do
918                         gun = g
919                         break
920                 end
921         end
922 
923         if state.enemy_sensor and gun then
924                 enemy = car.surface.find_nearest_enemy({
925                         force = car.force,
926                         position = car.position,
927                         max_distance = gun.attack_parameters.range + 3,
928                 })
929         end
930

I would change the enemy sensor code to

923         if state.enemy_sensor and gun and clip and not clip.is_empty() then
924                 enemy = car.surface.find_nearest_enemy({
925                         force = car.force,
926                         position = car.position,
927                         max_distance = gun.attack_parameters.range + 3,
928                 })
929         else    (Panic mode)    end
930

(If present, the ammo sensor would have loaded the weapon, so trunk is empty and can be neglected.) I'm not sure if that's necessary, but just to be on the safe side, I'd move lines 931-941 before line 923. This way it should be certain that no ammo is left in the trunk before the enemy sensor decides that the vehicle should go hunting:

931         -- reload
932         if state.ammo_sensor and gun and trunk and clip and clip.is_empty() then
933                 for item, count in pairs(trunk.get_contents()) do
934                         local stack = { name = item, count = count }
935                         if clip.can_insert(stack) then
936                                 notify(state, "reload")
937                                 trunk.remove({ name = item, count = clip.insert(stack) or 1 })
938                                 break
939                         end
940                 end
941         end

What do you think about this?