Found my bf keeps a note from ex

Hi ladies,

I was recently staying over at my bf’s house while he was at work. I am studying from home so I was alone all day. I needed a notepad for an assignment and went looking for one in his shelf. While doing so, I came across a note/card my bf keeps from his ex. I was so sad and hurt. My first reaction is maybe he didnt realize he still had it. But the note was sweet and loving and its obvious he kept it for a reason. I dont want to confront him because he will say I’m overreacting. But, should I be concerned? I never keep anything from any ex, good or bad terms. I should add, the note wasnt in plain sight like he looks at it everyday. It was tucked away in a compartment of the shelf with several other miscellaneaous notes and papers.