A new Nearby feature is now starting to roll out for Pokémon GO via the Adventure Sync functionality. This feature allows players to find nearby Pokémon even while the app is closed. It is now live in multiple regions, including the U.S. and Canada. Read on below to learn more:
Enable Adventure Sync: Nearby
Adventure Sync can find nearby new Pokémon even while Pokémon GO is closed!
Pokémon Blog 