Finding when a member viewed a thread

[Timelord]

I have had a problem very recently on my small forum where a "spy" viewed somewhat sensitive information that is only visible to registered users and reported it to someone else. I am able to view a list of the users who have accessed the thread and I am attempting to narrow down the list of suspects. I have a relative time frame in mind -- is there any way to determine when exactly certain members have viewed the thread?

Also, can anyone else think of any way to narrow down the list of potential suspects?

Thank you very much.