Where would the developers find the sounds?
If you hold it, the tone will play out the full length. So if you lightly tap it, it will make short sharp sounds. But this keybind (unlike the others) must be held down in order to keep the siren on. To turn the system on you'd press E, but a script could automatically have the siren disabled, so that only the emergency lights come on.
But it'd also work if you've enabled a vehicle to have siren and lights if you type /siren, e.g. This would work if you were using a vanilla emergency vehicle, e.g.
