What Did I Find...??

Okay so my SO and I moved into our very own home in July. We went through every box together from his room and I saw this box with all of his childhood photos, yearbooks and baseball pictures (absolutely adorable btw!!) and so yesterday I was cleaning out our closets and I came across the box again and I wanted to look at them again and well, I found an envelope (that was NOT in that box when I looked at it in July) with his ex's pay stubs LIKE 30 of them, her SSN and her USB chip thing. I'm just confused (not really mad) at the fact that this was in there now and not when I looked in July. Any ideas as to why it's in there?