Really? Whatever may or may not have happened 30 years ago is irrelevant. Look, she has given you no reason to doubt her in the last 30 years. Why get hung up on something that you think might have happened just because in an intoxicated state she did not respond to your inquiry?
Let it go already. It has been 30 years. If you were talking only a year or so ago, my answer would be that the two of you really needed to sit down and talk about it. But it was not recent. And, per your post, nothing has thrown up any red flags to you for the last 30 years. If you don't drop it, you will only damage your relationship going forward. And for what? What MIGHT have been a ONE TIME unspecified indiscretion?