I don't think he means open questions, but things related to the problem.

I interviewed once with a company that OCRed medical documents. One guy held up a sheet of paper and asked "How much memory does this take up?" which was a deliberately open-ended question. I was initially stumped, but then he started feeding me domain knowledge and assumptions, such as common DPI values and expected fidelity. Then I started coming up with estimates.

I don't think he expected me to have answer right off. I think he wanted to see how much I could figure out on my own if given hints.