That's what I'm wondering, too, acanthus. But what if it were just left out by error? Or for SIL to find, not you?
That's why I still agree that you should say nothing, etc. Unless, maybe, you just start talking about marriage, one day, somehow, and see if she says anything. But choosing the time and the exact topic (problems in your own marriage when you were young, a general conversation or whatever) might be difficult. So maybe not.
Really, she's a grown woman and if she wants to talk to you about something, I'm sure she will.