Find my works without a network connection. So if the phone is lost without signal and another iOS device comes in range it will remember seeing it, and then upload the location to apple for the devices owner to see when it has internet again.
Oh really. Wow that's pretty amazing. I thought it worked by piggy-backing off the connection of nearby devices meaning that those nearby devices would still need to have an active network connection while near to your device. I didn't know that it would remember the location of a device and then upload that location later on.
