Plot hole: While the pair are duelling on Kajimi, Kylo Ren only realises Rey is onboard his ship when the mask of Darth Vader is force-teleported between them. However in this same fight Rey is wielding the dagger, which Kylo must have known was also inside Ren's chambers before the fight. So he should've been able to determine Rey's location earlier.
Suggested correction: While he should have had some idea, it doesn't prove she is in his quarters, only that she has been there. It's also a small item that he may not have noticed in her hand, focusing more on her lightsaber.
Hogwash. He could have easily noticed it, even if he was focused on her lightsaber. Maybe it is small but rather easily noticeable.
I think the biggest point is the mask proves her location whilst the dagger in her hand doesn't.
It's also a weapon. In a combat situation a trained fighter like Kylo would have checked her other hand for a weapon as it is a common tactic to check a swipe with the main blade and stab with an off hand short weapon.