For those curious this seems to be how James Renner found out about this. He says it popped up in an Amherst, Massachusetts police blotter.
https://twitter.com/JamesRenner/status/1391384297777549314/photo/1
So it looks like whoever the person is did bring it to police specifically because of Maura. It says she thought the notebook belonged to Maura. If it did belong to Maura there might be some suicidal thoughts in there or perhaps thoughts of just disappearing.
But then again the way this case has gone it will be from Maura and all it will be will be about her talking about what party she is going to go to over the weekend.