Seems impossible and weird. Stealing money from a bank account is acceptable thing but from website ? Yes, that is true and every web master should take care of this.
When a hacker hacks a website, he tries to take website down or redirect. But some hackers don’t change anything in website which is visible. All they change is adsense publisher id.
The change is so slight that webmasters hardly notice it unless they see source code of his website, that not everyone checks frequently. Replacement of adsense code can be noticed only if webmaster is aware of his publisher id else one won’t even notice that he is hacked and money is going away.
How to prevent your site from such hacks:
Wish you long live webmastering !