Airplane mode cuts off all or non-wifi communication, which is undesirable. Receive-only mode would allow receipt of texts and recorded messages, save power and prevent detection of the phone by radio frequency methods (not by a metal detector). If the phone is stationary, then there is no need for it to send periodic keep-alive or hand-off signals to the cell phone tower. The phone’s accelerometer and GPS receiver can detect with reasonable accuracy whether it stays in the same cell tower’s range. Only when the phone moves a large enough distance will sending hand-off or check-in signals become necessary.
Location can also be detected using the radio receiver of the phone (which every phone has for calls and texts) if multiple cellphone towers are in range – just triangulate. A saved map of tower coverage areas in the phone helps position the phone and detect when the phone moves to a different tower’s area.
A software modification should be enough to create a receive-only mode: turn off sending (supplying power to the antenna) but keep receiving (measure and record the voltage and current in the antenna). Add optional deactivation of the receive-only mode based on the accelerometer and GPS detecting the phone moving out of range of the current cell tower.