Found

I liked this book written by Margaret Peterson Haddix. Without warning, a plane materializes out of thin air and pulls up to an airport gate. No adults are onboard – just a baby in each of the plane's 36 passenger seats. And after the babies are removed, the plane vanishes as mysteriously as it had appeared. Thirteen years later, Jonah meets Chip. Both were adopted as infants, but Chip's parents never told him. Unbeknownst to the teenagers, the FBI has done everything in its power to cover up the bizarre landing during their early childhood. But cryptic letters start arriving in the mail telling both of them that they are missing and are being sought out.

This is the first book in The Missing series.