While we don't know who could have gotten into Mike's bedroom through all of the security inside the house, we do know of one person who definitely was in the bedroom - Anthony. I believe that when Frankie gave him that $100, he could have asked him to open the drapes to Mike's bedroom when he goes in. Anthony left that drawing on Mike's bed. There's too much coincidence for me that when Frankie meets the rosato's that he gets handed $100 bill outside the bar. He says something to the effect that he didn't like it. We do know that Danny Aiello ad-libbed " Michael Corleone says hello" when he is garroting him, Mike could have found out from Anthony that Frankie gave him the $100. Don't forget Frankie told Anthony " dont't tell your father about the bill, I don't want to embarrass him".