I would have to say that's probably insecurity from previously being cheated on by a man she was in a 3 year relationship with and a man who apparently loved her so much he proposed to her. I was cheated on and it took my ages to fix the insecurities it left me with. I'm just lucky my now boyfriend was able to put up with my paranoid ways because I was beyond paranoid.
I admit I've snooped before (not a phone, e-mails and letters) when I was in my twenties. I've never caught a boyfriend out cheating although I suspected it. I am insecure in other ways though.
What I did learn from it was that you will always find something that will hurt you. And it is probably totally innocent and can be explained away but, if you're looking with that mindset, you will read something into anything.
I would NEVER do it now and haven't for many, many years. It's sad and worrying that CF still felt the need to despite being a 40 year old woman.
Drugs and alcohol can also make you paranoid.