They do it. They find something when they aren’t expecting to and in an attempt to save it, they tuck it in a safe place out of the way quickly. The problem is that sometimes that safe place is a logical one and sometimes it is not.
And because they do it, when you go through a relative’s papers you should go through everything one sheet and one page at a time.
When we went through my great-aunt’s personal family papers, tucked in between two pages in the back half of a photo album was her brother’s discharge papers from the Illinois National Guard. The photo album was only half used and the discharge paper was discovered after turning from one blank page to another. There were pictures of her brother in the front of that album and it made sense to put it there. But if I had not looked through all the blank pages I would not have found it.