The main character is blind. The bad guy is going to fool her for a few seconds by lowering his voice and making it rougher, like someone who is getting over a cold. But she'll catch on when she smells his distinctive scent.
The smell needs to be unusual enough that she doesn't smell it often. The bad guy is kind of a low life. It could be pot,but he also has bad guy friends who would also smoke pot. Same problem with smoking or the smell of meth being cooked (it's supposed to smell like cat piss). Some kind of alcohol that is more unusual (not beer, in other words) would work. I thought of dandruff shampoo, but he's not the kind to car a ton about his appearance, although he does take showers, if notecessarily every day.