I have years of experience with federal background checks...they have the ability (and desire) to look at almost anything they want, especially "expunged" records. Depending on the scope of the check, they could also probably find an arrest that did not result in further legal action. It's complicated, but I could explain how it's done if you want.

Whether or not they pull out all the stops on airline pilot background checks I don't know, and neither do you. It's pretty dangerous to deny that something happened especially if they ask the question in a vague manner. Like I said, it is very, very common for pilots to get fired during ground school when the federal check comes up positive for something the guy "forgot" to mention at the interview. And the worst part of this is you have now been fired from a 121 employer for falsifying your background...you will NEVER, EVER recover from that one!

Personally I think it would be better to admit the problem at the interview, get declined by 9 employers and hopefully get hired by the tenth than to risk getting fired for non-disclosure.