Yes, he's hiding something. His behaviour is easy to read. Question is, what.
It's from ten years ago. Before you met. He has a past! So do you. Don't we all.
It's likely it's intimate pics of an ex. She has a right to privacy. He's probably deleted them now. He's considering her privacy. This is a decent thing to do.
He hasn't looked at it in ten years. He'd forgotten the pics were even there. All good signs - it's the PAST. You're the present and future.
But all that doesn't change your lingering anxiety there might be something less ok.
Sadly devices may be used to store pictures that get men into trouble and not just with their partners.
You have a choice, now. Either accept it was the first possibility and let it go.
Or you calmly and quietly tell him you know him so well that his body language, his manner etc broadcast that something was wrong, and that you are fully aware he didn't want you to know what was on the iPad.
Tell him you need openness and honesty on your relationship and ask what it was.
(I wouldn't start with ‘its ok if it's saucy pics of an ex’ because that gives an easy out to seize on).
Just have a calm adult conversation.