your kid moved it

My baby is only 10 month old. He can only crawl. But he moved one of my hair pins from the hair pin Box to laundry basket.
My parents in law look after him amlost every minute.

The hair pin Box is put on a book shelf that too high for him to touch. The laundry basket is 20 meters aways, one floor down.

Here is how he did it -- by our guessing.
(1) he shaked the book shelf. hair pin Box fell down. He hide some hair pins under the sofa, or in the corner behide desk legs.
(2) He threw the hair pin to downstairs. the hair pin either landed under the piano, or it roll to some place not noticeable.
(3) When he was moved down-stairs for his meal, when my mother in law was preparing his meal, he crawling around, and move the hair pin to the laundry basket which sometime put beside the stair for a few minutes when my hu*****and collecting our dirty out-wear jackets. Then the laundry basket was taken by my hu*****and to laundry room.

We did not exactly see how he did the hair pin, but we saw he did simular un-continuously actions, like shaking bookshelf until things fall down, hide toys, threw toys to down stairs and put toys into containers.

I am sure your kid was able to find that secret under the bed. Check places like vent-pipes, U-pipe of toilet, bath tub and sink where he might drop things into;
base-cabinnets, corner of draws, behind the piano where your hand is too big to go in, but a baby's hand can.