I hate that this is an acceptable interview question.
If it was any of your business I would have put that information in there. Or more diplomatic would be saying something like ‘I was caring for my dying relative, I left that information out because I wanted to include only the most relevant skills for this position.’
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.