At least on iOS, it doesn't access the GPS directly. It asks the OS for updates to the device's location with a certain degree of precision. It's up to the OS to blend various sensor data (GPS, cellular, Wi-Fi, Bluetooth beacons - the latter is used underground to get a good position fix without a line of sight to the sky) and send that to the app. If it was using GPS continuously it would drain your battery very quickly. So if the device can get a good idea of its location via Wi-Fi/Bluetooth, it will not even bother turning on the GPS.