@futurebird There's got to be a drop in something that can be measured.
Drop in current wouldnt tell you anyway. Yes it varies, but you wouldnt be able to tell the difference between two leds farther away and one led closer in.
If you specifically want to detect it through the main coil you cant do it without changing the design as far as I can think of.
The simplest solution would be to make the individual devices resonant with the main coil but resonant at different frequences. That would give you something uniquely identifiable to potentially key in on.