A vintage postcard in my paperback left me with a choice

I picked up a cheap romance novel and a faded postcard from 1972 fell out, addressed to someone in my town. Part of me wanted to mail it back, but that seemed like overstepping into a stranger's life. So I just tucked it into a plastic sleeve and put it back in the book for the next person. It's a fuzzy line between being helpful and prying into old memories.