True but if you're using a Adwords campaign your doing it most likely for commercial gain and in most cases people have valid domain ownership information entered for the domain. And even with privacy shielding Google knows the registrant, in case of a legal matter your details will be provided if the documentation presented is in order. And i believe, not sure though, since Google is a registrar themself they can view the information even without any legal pressure.
Domain privacy is not a tool for full anonymity which is a common misconception.
Someone needs to be very carefull in his steps to not tie their own identity to their commercial Adword campaign only to cheat the system... which is possible.
To tell you what could cause Google to find you depends on the steps you take or forget, and is not something i or other members on this forum would want to elaborate on
I know you're just curious as you mentioned but this thread is better of closed for the integrity of the forum to not give people any ideas or to have a in depth discussion how cheating Google would be made possible.
But that's not my decision and purely
