Husband looking up ex on IG

I was out with my husband yesterday and he wanted to show me something on Instagram. He went to the search bar and his history popped up and it was his ex gf. I saw it with my own eyes and he panicked and denied it. I said let me see and he quickly deleted it and searched something else. We've been married for four years, why does he feel the need to see her pictures? It's like he cant close that chapter in his life, is it because he misses her? Am I overreacting? It's not like hes messaging her, hes just looking at her pics...