For reference: There is a duell in the module where a shaman tries to secretly help one NPC by continously casting Cure Wounds on him while supposedly only helping him “stand up” after he intentionally let‘s himself fall prone. To spot this, a PC has to specifically declare checking for interventions a then suceed a DC 15 perception-check despite being within earshot…
Because it would make subtle-spell kind of redundant and why even use components in the first place if they cannot be used to tell you are casting something?