I don't think we are looking exactly at an unstated argument, but rather, at a potential weakness. We are trying to justify this fine on alarms, so what could telling us that burglar alarms are effective do? This does require a bit of creative thinking, but since we know that they are effective, then we certainly shouldn't ben them outright.
More generally, if a part of an argument seems to be preemptively plugging a gap, we could treat it as a rebuttal to a possible counterargument.
