On Sun, 9 Aug 2020 07:06:57 -0700 (PDT), wrote:

Please is there any gadget/circuit/App that can detect a mobile phone when ;
1. even when the phone is off but the battery is inside.

No. When the phone is turned off, all of the various transmitters
(cellular, Wi-Fi, BlueGoof, and NFC) are turned off. It might be
possible to detect the NFC (near field communications) resonant
circuit at 13.56MHz with something like a "grid dip meter".

2. When the phone is on

Yes, as long as "airplane mode" is turned off. There are
transmissions from the cellular section to the tower to allow the
phone to check into the network and establish its presence. If
enabled, Wi-Fi and Bluetooth also transmit keep alive signals.

At this time, power-on NFC services (such as Google Pay) intentionally
do NOT work on a smarphone unless the phone is turned on and NFC is
enabled. However, that might change as NFC begins to encroach on RFID
territory:
https://www.xda-developers.com/future-android-smartphones-nfc-to-work-disabled/
Meanwhile, looking for a 13.56MHz tuned circuit might work.

3. when the battery has been removed

No. Again, the NFC section may not require power to be detectable.
https://en.wikipedia.org/wiki/Near-field_communication
https://nfc-forum.org/what-is-nfc/about-the-technology/
However, the other transmitters require power to function. Same with
the various "find my phone" programs:
https://www.google.com/android/find

thank you.

Good luck. The big problem is not detecting whether there is a cell
phone present. It's how to identify a particular phone and how to
deal with the clutter caused by a multitude of phones in the same
area. It might be possible to detect a phone if it was the only phone
in the area, but not in a crowd of phones.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558