Have your publisher ID ever been used in other sites than yours, without your knowledge? I have never experienced the same problem but I guess some prominent publishers must’ve experienced this.
AdSense now have an Allowed Sites feature where you can specify on which sites your publisher ID will be considered active.
This is indeed a very useful feature, especially if someone decides to use your publisher ID with bad intentions such as to get your account suspended!
Read more about it in Inside AdSense: Allowed and clear.
To create your Allowed Sites list today, visit your AdSense Setup tab, click the Allowed Sites sub-tab, and choose the radio button for Only allow certain sites to show ads for my account. You’ll then be able to enter allowed URLs for all your sites. Make sure to keep your list up to date when you add your code to new sites, too.
So if you decide not to use it, no change is necessary!