Gun powder residue would be specific to recently fired weaponry so if there's ever a time Eggsy does go firing one off Drew would be able to pick up on it (and side-eye him really hard)

With reference to JB that'd be a smell that lingers so he'd pick up on that, yeah.

And lastly re: nightmares/emotional state stuff that'd be a thing he could totally pick up on. Even things like exhaustion/anxiety etc have a scent and I imagine if he's having nightmares it'll also show on his face in a way Drew will see it with his enhanced eyesight so this will all work.

Thanks!