Marinette noticed things that differed from Adrien to Cat Noir. Sure, they acted different sometimes, the mask helping Adrien become more confident or suave. But there were some other subtle differences Marinette noticed over time, as well. Cat Noir was more prone to sudden decisions. Adrien was more careful when discussing romantic things. Cat Noir spoke clearer, but that might just go with the confidence.
Among those things, were teeth. When Adrien was transformed to Cat Noir, his canines became far more prominent, giving him the appearance of feline fangs. It just went with the whole 'cat' appeal. For a while, Marinette thought nothing of it, other than it was a pretty cool costume effect. But suddenly, those teeth had a whole new meaning.
It started with just a simple kiss after an Akuma fight. They had been dating for a while as their regular identities, but when they were transformed, they but most of their time into fighting the Akuma. They didn't think much of the kiss, since they were fine with the world knowing they were together, if anyone asked. At the end of the kiss, Cat left a tiny bite on her lower lip. Adrien had done that only a few times, so it was surprising that he did that so quickly as Cat. Must be one of those little differences. But the thing that really surprised her, was that feeling of sharpness. Adrien's teeth were normally so perfectly aligned, being a model and whatnot. So the sharp teeth of his transformation were a strange anomaly.
She noticed it again, when Cat decided to visit her late one night. Unplanned, but not unwelcomed. Marinette welcomed him with a much more intimate kiss than the one after the Akuma. She wasn't the bravest soul in the world, but she wasn't nearly as shy as when she started dating him. And when her tongue brushed against a feline-sharp point, a whole new sensation was introduced to her.
But the most noticeable time, was after a significantly harder Akuma fight, when they were both emotionally and physically exhausted. They both needed something to relax them, and as luck would have it, their partner was exactly what they needed. And in a moment of rush, Cat had taken the opportunity to give Ladybug a bite on the neck, right where her costume ended and her skin began. Adrien did that only a small handful of times, but it was enough for Marinette to memorize. But this time, this time was with fangs.
Now, Marinette can't stop thinking about those darn teeth.