My answer is different than it was going to be from just reading the subject line.
In this case since it involved a roommate situation I agree with the comment above that over time one could make such a mistake, especially items as you said were not of any real value. If it's something you want tell her nicely that she mistakenly packed it. If it's not something you really want I would just let it go.
Now responding to the "Subject line"...
As I have gotten older I have whittled my friends down to a tight knit group that I would never have that problem.
