What was in the note? Only Jenna Fischer and John Krasinski know.
In the Office Ladies behind the scenes podcast for the episode, 'Christmas Party', Jenna Fischer and Angela Kinsey discuss the note:
Jenna Fischer: I'm gunna say something I've never said before. I also feel like I'm maybe gunna cry.
Angela Kinsey: What? Wait. Wait.
Jenna: What?
Angela: Are you gunna say what was in the note? Are you gunna say what was in the note?
Jenna: Sort of.
Angela: Because I don't know what was on that card, none of us do
Jenna: I'm the only one who knows, and John knows.
Angela: Oh my gosh, lady.
Jenna: In that episode in season 9, I believe Greg [Daniels] suggested to John that he write a personal message from himself to me, just saying what our time together on The Office meant to him.
Angela: Aww Jenna.
Jenna: Because we were wrapping up filming, we were wrapping up the series and we were all so emotional at that time, you know?
Angela: We were all sort of saying good bye in different ways.
Jenna: We were. And so, that was his [John's] goodbye.
...
Jenna: It was the sweetest note and you know on camera, Pam says, "I'll never say what it said, but just know it was perfect". We'll, I'll never say exactly what John wrote, but just know that it was perfect.