On Sun, 9 Aug 2020 16:07:57 -0700 (PDT), wrote:

My next problem is after detection, can the phone be zeroed in on? (located)

No, if you're asking if the GPS in the phone can be interrogated.

No, if you're trying to do direction finding. The likely problem is
that with the power turned off, the only thing that might be usable is
to direction find on the RF energy being absorbed by the 13.56 MHz NFC
loop antenna. I can think of few ways that might be possible, but the
range will be very small, and due to the low frequency, accuracy very
poor.

Yes, if the phone is transmitting on cellular, wi-fi, or Bluetooth
frequencies. Unfortunately, there is no idea system. The problem is
that there are other users also transmitting on cellular and Wi-Fi
frequencies. You not know which frequency is being used by the phone
being located and you don't know when it will be transmitting. Instead
of a nice clean direction indication, you'll see a jumbled mess of
users, frequency hoppers, spread spectrum junk, collisions, and
interference. You could build something that would work under
idealized conditions, but in a crowded RF polluted environment, it
will be very difficult to use.

Offhand, I think the best approach is also the simplest. Build a
rotating directional antenna on the frequencies of interest. A polar
display will show both the bearing and a radial line for the signal
strength. In other words, an AM (amplitude modulated) receiver. Use
an SDR spectrum analyzer for finding the signals coming from the
phone, and then switch the SDR receiver to the frequency of interest
for direction finding. Such an arrangement is far from perfect and
there will be situations where it doesn't work, but it's the closest
and easiest DF system to build.

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