I will be mentioning it to her this morning. There won't be a row - that's not my style - but I do want to make it plain to her that it is not on.
4 of us share 2 computers and we do not have the benefit of password protection, but I would have thought that it was an etiquette thing as much as anything. It seems to be a very rude and disrespectful thing to do and I have never come across anything like it before. (The person in question is new to this department, having worked elsewhere in the business up till recently.)
Thanks for the advice guys!