if Sam is hiding in the pocket, fully hidden and unnoticed, but Dean is chatting with another human, will the knack be mildly active due to the proximity or turned off completely?
Sam will, but only under certain conditions!
The knack is wholly psychic, so it operates without him actually needing to be seen. If someone glances at Dean’s pocket, he’ll feel a slight tingle and know to stay still. If someone actually knows he’s there, it’ll grow much stronger. The strength of the alert he gets is related to if someone has actually seen him and if they mean him any harm.
As it is, Sam will most likely elbow Dean to let him know whenever that happens. This way Dean can be sure to keep their attention off his pocket, or leave the situation if they’ve figured him out.