Do these emit something unique that could be detected so that loud klaxon could let everyone know there is a glasshole approaching? Some unique bluetooth identifier perhaps?
Yes actually. You can poll for nearby Bluetooth devices and cap the packets advertising the manufacturer.
Advertising packets only go out on startup and pairing, not during normal operation.
This may be true for standard BT (I genuinely have not looked into it), but Meta's glasses are pushing out ADV frames every few seconds, paired or not because they act as a BLE beacon.