SoHK presents that as an optional rule, and suggests that if the rule is used, you also increase the chance of detection (spell risk). The core rules don't say anything about it. I think the default assumption would be that spells make as much noise as their effect (e.g. a lightning bolt is loud, but a detection spell may have no outward detectable aspect), so in many cases the spell is only detectable if you are speaking, gesturing, etc in order to cast it. RMSS/RMFRP give modifiers to the spellcasting roll for casting silently, without hands, etc. I think there is an optional rule in RM2 about this as well.
Bottom line, though, is GM's choice.