I think that a security person, including Rocco, would more naturally be present around Michael's bedroom than Fredo would. Buildings don't get secured and then are left alone - that's why modern buildings have cameras, instead of just a guy at the desk. Someone may have slipped through or been concealed, so security makes their rounds.

In their very emotional confrontation, Fredo only tells Michael useless information. They've got Pentangeli. He bumped into Johnny in Beverly Hills. Questadt belongs to Roth. From that, you can either construct a series of musts to solve the mystery, or you can accept that there are things you don't know and move on to the drama between the brothers.

I haven't said this in several years, but if you want to solve the "mystery" of the story then Bussetta serves about as well as anybody. He could kill the shooters, he must have been around the compound because he's right there on the train with Michael when they return to the contemporary story, and when he kills Johnny they show him emerging, ghost-like, from a set of drapes. Again, I point this out not to assert its truth, but just to show how loose the script is respecting these matters. You can make a case for anything.