I don't think theres currently a way to find out if someone other than you has been reading your PM's, sorry Remus.
If you think someone else has got hold of your password and is logging in as you, you could ask the admins of sauk to search their board logs to see if someone else has been visiting their board logged in as you though
Also, it's a good idea to change your password asap just incase someone has got hold of your password
and do moderators on boards have access to your PMs?
Nope, admins & mods cannot view their members PM's