I am trying find a way to scan for the Bluetooth id's of mobile phones even when the phone's aren't visible/discoverable. I am assuming that this is possible because if I have a phone and a bluetooth headset which I previously paired, but at present both are not discoverable, they will still find each other. If by making the devices not visible that meant they stopped broadcasting any identifying information, then it seems it would be impossible for two paired devices to find each other when they're not visible. However, I know very little of the low-level details about how Bluetooth works; I'm a software engineer working on Linux with vanilla USB-Bluetooth dongles using the open source bluetooth stack.
I understand that the solution may require some custom firmware in the bluetooth dongle or other up front work, and I will be able to get a budget to fund that work if somebody knows how to do it.
Thanks.