There's two ways this can play out. Either she's wearing a new perfume, or she's picked up the light scent from another man. Neither of them are good news. The first, like new clothes, is a sign she's ready to start flirting again. The second is even more obvious: it means that she's actually been close enough to a guy for him to rub off on her. Let the mental images begin!
It really is a dead giveaway, so it's kinda unfortunate that it doesn't happen as much as it does in the movies. First off, she's not going to be bathing in that guy's aftershave, no matter how much she likes him. Secondly, it requires the nose of a police beagle to even tell the difference most days. Still, keep a nostril out for any changes. And if she starts smelling like weed and burritos, marry her.