Let’s say you’re trying to find hidden cellphones in rooms full of bunk beds with metal frames.
Option 1 - physically go through everyone’s belongings, which would be extremely time consuming.
Option 2 - use a metal detector, like those wands they use at the airport. Would the fact that the bed frames are metal make this a futile exercise?
Option 3 - an RF detector, which only works if the cellphones are on, so then you’d have to pace the hallway waiting for people to start using their cellphones. I don’t know exactly how these work - would it be possible to tell from the hallway in which room the cellphone was being used?
How do you know the number? I suppose you could ask the people that are hiding their cell phones what their number is, but I wouldn’t count on getting an answer.
Pretend the sprinklers were just set off, or the mattresses are being changed, or barracks being switched, and watch to see who runs in to grab their phone!
If the phone is completely powered off (i.e. battery removed or some such - not able to receive calls) then it’s just a matter of moving all the inmates out and tossing the place. There’s nothing detectable about an inert piece of electronics IIRC.
If they are on, then I assume a radio detector would work. Not sure where those are available - simple solution would be to have one with adjustable sensitivity, do a sweep with it able to find something at, say, 20 feet, then dial it progressively down while sweeping the area. A live phone is IIRC pinging the cell tower every second or so to ensure the cell knows where to find it for incoming calls.
Actually a live phone can be silent for hours. Once the phone has powered up it listens for cells, and then engages in a setup protocol with the tower. It then “camps”. It is the phone’s responsibility to work out if it is moving into range of another cell, and to initiate a handover. Otherwise the phone will sit quiet for hours, before making a few second “still here” noise.
Finding a quiescent phone is about as hard as finding one that is powered off. Unless you can force it into a renegotiation - something that might be possible - but would almost certainly involve use of an illegal transmitter.
Locating an in-use phone would probably be best done with a set of three receivers that perform a time based correlation of what they hear, and allow triangulation of the location of the transmitter. The digital transmissions used by phones are perfect for the job. However this isn’t a simple off-the-self solution. Would need reasonably wideband receivers, and some significant post processing.
If someone was hiding use of a smart phone, and were stupid enough to leave WiFi enabled you could probably set up a base station that entrapped the phone, and then go sniffing for the WiFi signal. At 2.4 GHz (or better 5GHz) you can get a nicely directional hand-held antenna to go sniffing with.
Depends on how much you control the owners. i.e. what you can do with CIA secret prison inmates is more than what you can do with Girl Scout campers.
Take each person’s belongings and dump them in a trash can full of water. If a phone is not in a waterproof container it’ll be inerted. If it is in a waterproof container odds are there’s enough air in there the thing will float and be easy to find.
To be sure if the inmates know this is your search method they’ll develop a countermeasure.
Another idea is to run all the belongings through a microwave oven for 5-10 seconds. Once again any phones will be inerted.
Not strictly true. An unpowered cell phone still contains semiconductor junctions attached to antennas. If you excite one with a strong electromagnetic field, then the nonlinearity of that semiconductor junction may create energy at harmonics of the exciting frequency. That’s detectable, and relatively distinctive, so it’s a classic way to sweep for bugs.
Cue the story of the Russians embedding thousands of diode-plus-antenna circuits in the concrete for the US embassy, rendering that technique useless due to false positives…