If sharingan wasn't always active then Kakashi wouldn't need to cover it, he could just deactivate. Also, I think that Sasuke needs to activate his because it's not complete.
Kakashi and Zabuza sensed Naruto's Chakra when he went crazy on Haku, and Kakashi could tell that Naruto was stronger than him even before that. If Orochimaru is more powerful than the Kage's, then he would be oozing power and everyone would be able to sense it, no? And even if they didn't go "oh, that must be my main man Orochi" they should have at least mentioned how strong he was.
EDIT: Plus, Kakashi was able to detect Gaara and know it was him, and I would think that Orochimaru's "murderous intent" would be MUCH larger than Gaara's.