Was he unreliable as a person before? Was he combative with his previous employer and colleagues? Was he actually in prison all this time? What if this guy is a corporate spy?

I’m not sure you will get that information by asking about a gap.

I believe it would be more acceptable to ask questions more specific to your concerns. “Can you tell me why you left x position?” Even then, you are not going to get the information you are so suspicious about, you will get a bullshit interview answer.