@FC,
Yes, you see the loc-signal swing from left to right and back. Most of the time you see it when somebody crosses the signal before you (when you are on final) but it can happen when you are on the ground as well.
To operate under Cat3 you still use the same ILS as with Cat1, but with many more requirements:
-redundancies in aircraft systems (a.o. multiple autopilots; multiple and separate electrics and hydraulics)
-redundancies in ground based systems (o.a. backup power; failure monitoring)
-last but not least: LVP (low visibility procedures) for ground movements. Somebody crossing the LOCantenae can easily disturb the signal; runway incursions by vehicles occur more easily. Therefore there are a.o. restrictions on the amount of movements and extra protection areas around antennae
LVP have several phases and as vis goes down, the requirements go up.
I'm just a pilot, perhaps a controller can give you accurate info
Now the tricky point: while modern aircraft can land in almost any vis, the airports effectively lock up during low vis; capacity is decimated. Hence airports very quickly phase back their lvp when weather improves and there you are: doing auto lands under marginal weather without the protections, but with the risk for a swinging loc-signal.
For me these are the most risky autolands, especially the auto-rollout.