Arab Strap: Love Detective

We slept in this morning and she had to get ready in a hurry - no time for her usual attention to detail - and she ran out the door, slamming it behind her, leaving her keys swinging and jangling. I stayed in bed until I heard the downstairs door shut, then peeked through the blinds and as soon as she was out of sight, I went for the keys. She never tried to make a secret of the box or the fact it was locked or even where she kept it. But as I said at the time - "If you've nothing to hide, why hide it?"
It's one of those wee red cashbox things and she keeps it in a drawer by the bed, under some pictures and books. Every key she has is on the same keyring - it took me a while to find the right one. I don't know, I suppose I've had my doubts for a while. There's been hushed phone-calls virtually every night, her friends stop talking when I come in the room and they look at each other, and I don't know, it's just a feeling. Anyway, I eventually found the right key and it fitted perfectly in. I put the box on top of the bed and opened it up...
There were these pictures of friends and ex's, letters, postcards, doodles, nothing bad - and then I found some sort of sex diary and I went to the latest entry. It explicity detailed a recent adventure up the park with a boy she said she had forgotten about...
And it got worse as it went on. The dates never made sense, there were people I had never even heard of. Eventually I had to stop reading it because I started to feel sick. So I put everything back the way I found it, shut the drawer and phoned you. See, I don't know what to do. I keep having fantasies about leaving her dictaphone under the pillow or following her when she goes to work. I've been lying about where I'm going, just in case I can bump into her.