That only works if the devices report WHO saw the missing device. If they both report up that the device is in the area ask but don’t say who is making the report then you can’t figure that out.
Apple sees the reporting device’s ip-address. Obviously Apple is in an excellent position to spy you anyway, but the claim that even Apple doesn’t know where the turned off device is doesn’t hold for the reporting devices: Apple can infer which reporting devices are in proximity.
And they possibly even know who the reporting IP is because of iCloud.
But reporting device A and B were in Bluetooth distance to lost device C. Therefore A and B were close together (like a few meters). It’s a huge improvement over A and B were in the same mobile cell.
You can then use the IPs to identify who are A and B.
