Some mobile devices deactivate Wi-Fi when in standby, iOS was always notorious for this. So bluetooth may be a better option.
But if you do want to scan for Wi-Fi connected clients, it is much easier if you are all on the same network (e.g. the Pi and the phones connected to the same AP). Then you can simply run an arp scan
sudo arp-scan --localnet --macfile=/home/pi/mac-vendor.txt
In this example, I've also given a vendor file which lets arp-scan return meaningful names for known devices.
Otherwise, you should look at some Bluetooth utilities that will let you scan in a similar way though I warn you that the intricacies of Bluetooth make it fairly horrible to work with in my experience.
A tool such as Node-RED may be helpful to let you do the scanning and reporting without loads of programming. The Node-RED forum has plenty of people who regularly do this kind of thing for fun.