Actually, Mr Dabbs
Many companies would consider that to be 'unauthorised use of a computer that's not yours to use', and hence subject to disciplinary procedures.
You have no idea (or perhaps you do) what a sh**storm you can cause by sending an email from that person's computer (like a colleague did) or doing something innocuous as the above, regardless of whether they were supposed to lock their desktops or not. Employers take a very dim view on that kind of behaviour.