Sasuke always knows when some girl has caught Naruto's eye (because he is always watching Naruto) because in spite of being two years a ninja the idiot still has no sense of subtlety and likely never will.
To see is to perceive; whatever nuance attracts Naruto's interest he deconstructs and incorporates in Saki-chan, or when he can't, he makes inconsequential in light of Saki's intrinsic charms. To Sasuke's grudging satisfaction, Naruto never has eyes for anyone but Saki-chan when she is out in force. (He sees it as a job well done.)
But Naruto still looks when she isn't. And it is less and less often now that Sasuke can use what he sees; either Saki-chan is approaching Naruto's ideal, or her archetype is holding her back, or Sasuke is. And when Naruto is forever running off to spar with the other boys in their graduating year and the one above
Sasuke is entirely aware that as a friend he is sub-par. But Naruto is his closest friend, and he needs (to warn him) (to disarm him) to let him know. And so, Saki. And so the kunai in the heel of Saki's left boot, and the throwing star in her right. Because (to achieve his one ambition) he puts too much effort into this farce to be snubbed for the soft civilian girls Naruto always drools after.
(Sakura, strangely, was the only of her ilk.)
Saki can't get at her weapons with ease, but they are in the only place they could remain reliably hidden. With what Naruto does withanyway, it is a misfortune of (accident) (design).