(See in situ)

Smartphones intermittently send data packets that will register with nearest tower & pick up on your location.I haven't heard of the program that recommends friends on FB.That's a new one on me.But with a smart phone,being used or not,they can pick up on where you are.And with any cell,if they(authorities)have your cell#,they can locate you whether it is on or off.So is a good idea to leave it at home now & then.
The things that can be done with a smart ph is why I no longer have a smart ph.It is a handheld computer that can be hacked over the network.I'll never have one again.