I can assure you they do exist. I have bought a new one from Citroen before now and fitted it, as well as transferring another one from one radiator to another. My cars were/are a March 88 (14RE) Jan 91 and March 91 1.6 Athena(s) and all have this type. To check the switch I might put a battery back in the engine empty one and plug in rad then put a magnet between the prongs.
The 'float' I had was a bit different to the one shown in the Haynes but same principal, flimsy hollow capsule with 2 grooves at the bottom. Basically you spring the prongs of the switch to engage on the grooves. It is more than conceivable that it could be knocked free in an accident. But where did the other one go? Maybe the antifreeze has dissolved it, it really is a bit odd. Maybe the sea air on the way to 45° South dissolved them!.
It's easy enough to install, its just catching half the coolant so as not to waste it thats the fun....