That is one of my pet peeves. After markers, crayons and gum going through the washer and dryer (that was a mess with gum) I took duct tape and taped the front and back of the washer cover and wrote "check pockets" .
I usually always check their pockets. When the above happened husband did the wash. I got stuck with the clean-up.
My rule is - - - whatever is in the pockets when I check becomes MY property. If it is money, it is mine. If it is a Note - school/friend note - I read it. Both my kids grumbled about that. I just told them if they don't want me to read it, check your pockets. It is MINE once it hits the laundry basket down the shoot.
That tells you how long this has been going on. easy child is 26!
I had a jar for all my money I found. Then I noticed my jar was going down. So I have a secret hiding place. I bet I have $100 in there now. Someday I'll go buy myself something, on them!
It isn't your fault. That is one area I don't bend on. It is there fault. It drives me insane.
Other sign I put up (really tacky) is I put that same duct tape on the inside of the toilet lid and wrote FLUSH. Didn't work either.