I'm reading the source for the nRF Toolbox app for Android 4.3 and comparing it to the Proximity Profile spec from the Bluetooth SIG, which has this to say about path loss and alerting:
The Proximity Monitor shall maintain a connection with the Proximity Reporter and monitor the RSSI of this connection. The Proximity Monitor shall calculate the path loss by subtracting the RSSI from the transmit power level of the Proximity Reporter as discovered using the Reading Tx Power procedure. If the path loss exceeds a threshold set on the Proximity Monitor it shall write in the Alert Level characteristic of the Immediate Alert service, using the GATT Write Without Response sub-procedure, to cause the Proximity Reporter to alert. The Proximity Monitor may also alert when the path loss exceeds the threshold.
I can't see any such subtraction of RSSI from the TX power level going on in the Android app. Does that mean the app is not a strict implementation of the Proximity Monitor requirements in the spec?
Is it even possible to implement this reliably in the current Android BLE stack? Once you're in a connection, how would you keep measuring the RSSI?