This is actually the closest guess to his actual knack that anyone's gotten, so we're gonna give it to you!
- Attention Manipulation: The ability to psychically manipulate a human's attention.
Stan's knack works a bit similarly to John Winchester's, in that he is able to divert humans' attention from himself if he focuses on it. In the case of the scene you mentioned in Live and Let Die, Dean was a bit distracted doing what he was doing on that hunt, but Stan was definitely putting out 'don't look in your pocket' vibes that sealed the deal. At least for a bit.
While Stan concentrates on not being seen, humans' eyes won't focus on him and/or they might ignore his general vicinity. Even if he were out in the open and a human tried looking right where he was, with his knack active their gaze would simply slide off him like... uh, whatever it is water slides off. It's an incredibly useful skill for a borrower to have, and even in AU like Brothers Chosen where he doesn't realize he has it, he can still use it!
And though it isn't as easily discovered by him, his knack also works in the reverse: Stan can draw a human's focus towards himself, making it difficult for them to ignore him or even look away if he's really digging in. This could be a handy tactic for diversion, or a way to alert a friendly human that something's going on with him.
Like most knacks, Stan's does have its limits and rules. It works similarly to Sherlock's knack, where it only affects Stan at its base level but he can train it to affect others in a larger area with much greater effort and higher chance of burnout. It also doesn't affect borrowers/littles' attention at all, so even if he's fully hidden from human view, a fellow smol will be able to see him just fine.