Why did my friend steal of me??? please help?

on tuesday my friend and her son came round to chill with me. We are not really close but we are friends and have known eachother for around 6 years. However, when she came up I explained to her how I was off to see my husband who is in prison at the moment etc. I did some cooking in the kitchen and left her in my bedroom on my laptop, my handback was on top of a set of chester draws next to where she was which I had left open, as before she came I put in everything I needed for the visit, including the visiting order, my ID a couple of books etc which I know I put in before she came, however when she left I had a funny feeling to check my bag, when I did the visiting order was not in my bag. I could not understand why she would take it I rung her and asked her if she had seen it and she said no I hope your not accusing me of taking it etc tc - I know for a fact she took it because I packed it all in the bag before she came round - why would someone do that???