The Mattis thing, and pickpocketing in general, could be somewhat solved by doing a simple Slight of Hand vs Insight or Perception check. Their (or Your) Slight of Hand vs the Insight or Perception of the party/individual. This could be done still in the turn based conversation where everyone is involved, and could give consequences to getting caught, though knowing how Larian handles NPCs it will almost always just result in a giant fight with no way to de-escalate the situation through persuasion/deception/intimidation or even some spells.
