Lead Developer
In your Dashboard, go to “Appearance > Editor” – you’ll see the header.php file in there that you can check.
If it’s been added in there, chances are you’ve been hacked.
If it’s not, try deactivating all of your plugins and see if it disappears. If it does, activate them one by one until it shows up again.
Are you using GP Hooks? If so, did you add the adsense code in the “Before Header” hook in “Appearance > GP Hooks”?