How to detect a phone

[Robert]

Is it possible to detect a bluetooth (BLE) enabled phone when it enters the room?
I like to use the nrf51822 (BLE-Peripheral or BLE-Central) to detect any phone when it is in range.
Preferably without any app running on the phone, but I have no idea if it is even possible.

I found an almost similar question here, but the answer involved changes at the phone end.

Any help is greatly appreciated.
Regards, Robert

[Dave_couling]

The phone has to be either scanning or advertising. This would require any app. You could have your device either detect a Scan Request if the phone is scanning, or have your device scanning to detect a phones advertisements. Either way you will need an app on the phone, with the App running in the foreground.