Greetings sleuthers.
...I ask because I was speculating otherwise :-D One possible explanation for (one of?) the handwritten note(s) was that Sam wrote it as part of a bumbling attempt at alibi. He mentioned to some driver or another that he waited for his girlfriend to fall asleep and left before she awoke. Perhaps that conversation combined with a phony goodbye note, ala "Sorry Emma, I love you but it's not working out. I had to leave thx for the hospitality <333333333" was meant to introduce reasonable doubt regarding his involvement in her murder.
Some have already speculated that his call to the police about spooky sounds in the basement might have been an attempt at misdirection as well. To be clear, I am not insinuating that this would be a sensible move; rather, it seems a pretty hopeless act. But he was almost certainly desperate at one point or another, and he might have become convinced that "it's just like I told that driver" + "scary sounds in the basement, after all if I murdered them OF COURSE I wouldn't call the cops" + "see, I even left a bittersweet goodbye note" was better than nothing.