80
You can really tell that they wrote parts of the module before publishing the rules…
(ttrpg.network)
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…
RAW verbal and somatic components give you away similar to making an attack. That's why Subtle Spell and similar exist. There's no "roll to cast secretly" mechanic on purpose, the benefit of being hidden is that you get advantage on your first attack