I have a bin storage system for the kids' toys. On the front of each bin, I attached a picture of several of whatever item goes in it. This started when Jessica couldn't read and Xander can't read yet, so we still need the pictures.
The picture on the car bin was taken a couple of years ago. It has a Fisher Price police car and Parents brand truck. We handed down the Parents truck a while ago, since it's more of a toddler toy. The Fisher Price car I just put into storage a few days ago. Really. He's got a playmobil police car that he likes much better and hardly ever plays with the Fisher Price one.
This morning, he came over to me, holding the picture he'd torn off the car bin: "I don't have this car," he said. "Where is this car?"
So it took him maybe 2 days to discover a toy he never plays with was missing.