Bonjour,
Both the S4 and the S5 have removable batteries. And I wonder if there is an independent battery for the modem that would allow it to be powered on even when the main battery is removed.
- The teardown of the S4 shows a Qualcomm MDM9215M modem.
- The teardown of the S5 shows a MSM8974AC (Snapdragon 801) which includes the LTE cellular modem.
Both are located on a board that does not seem to have an independent power source. But I know close to nothing when it comes to hardware so… feel free to correct me 
Any idea ?