The boss for a campaign i am writing right now is a high level spell caster. He will frequently be using the invisibility spell. The adventurers are all level 3, and none of them have any spells or skills that allow them to specifically locate an invisible object.
Is it possible that they can listen to the verbal part of a spell and locate where he is? I ask this because i notice that 14th Level rogues get the feature 'Blindsense' which allows them to detect any invisible creature within 10 feet of you so long as you can hear them.
Furthermore, by using a sorcery point, and investing it into metamagic, the boss can cast these spells without the verbal component as specified in the Subtle Spell section. If he were to do this, would there then be no way of detecting the boss?
I assume a high survival check would enable them to see footprints moving, but other than that?