It's happened before -- even to US citizens. And you can pretty much be sure that racist stereotypes held by CBP employees come into play in whom they ask which questions, how frequently and why they ask them, and the extent to which they may try to research the person or a subject matter related to a person and then use that "research" to ask additional questions or do something else that further delays the individual from being allowed to continue on their journey.

I've seen CBP ask Americans -- and I'm speaking of those with prima facie evidence of US citizenship and identity already presented -- about what is famous about listed home address cities on a declaration form. It's rather amusing when the CBP employee's ability to Google flops to try to verify information that may or may not be that esoteric depending on where the persons have lived, their interests/historical knowledge/age, and/or when and how competently/incompetently CBP employees use the search engines.