Hi, and thanks for your patience.
We don't currently log access to private messages. We're researching how we can implement access logs on our systems.
As Kate mentioned earlier, 'Staff with mod clearance are able to look at PMs, and they very occasionally do so if they believe that it's necessary in order to protect other users. Once PMs are deleted they can't be viewed or retrieved.'
We have tight control over who is able to access PMs and there's also a 'delete' option that you can use for anything in your 'inbox' or 'sent' mail that you consider too sensitive.