.
I support the argument that it was likely one of the extra security men that was brought in to the party. It's certainly reasonable to think that security would have access to the bedrooms. Seems silly that they would be forbidden from inspecting the house prior to Michael's return.
Another point in favor of the traitor being a muscle guy is that it also provides an answer to the other big question from that scene: Who killed the assassins?
.
Logically:
--If Roth wanted to have Michael killed in his bedroom, and...
--the drapes had to be opened so the gunmen could have a clear shot at Michael, and...
--a security guard was bribed or planted to open the drapes, then...
What would they need Fredo for?
