I grew up with a mother who would sneak into anything, diary, mail rec'vd from my girlfriend, you name it. I knew there was no such thing as privacy under the same roof.
Most kids, especially teens, have a cigar box or shoe box, whatever, for their little keepsakes, private notes etc. I learned quickly that my mother snooped in everything and a little tin cookie box in the back of my drawer would be checked regularly.
No, I never read diaries or snoop into personal spaces with my kids. We had a little girl toddler who snooped into her brothers and sisters stuff all the time, looking for chewing gum and life savers. She discovered the note from the 20 year old elders nephew to our 12 yr old daughter and innocently gave it to us. That was serendipity.