Back when I was a teen, I spent a summer at my Aunt’s house. I had my own bedroom with an attached bath. I was partaking at the time.
Taking a small screwdriver, I could loosen the set screw on the towel rack in the bathroom and hide things in the hollow towel bar. I thought it was pretty slick.
I also had an old car ('62 Olds 98) that had a armrest you could pull down in the middle of the back seat. With the armrest down, you could pull out a sort of flap behind it and keep things behind the seat.
Now this isn’t hiding, but I use an empty chap-stick to keep some extra blood pressure meds.
How about yours?
(not looking for hiding places, but thought it might make an interesting thread)