Well, they weren't that sophisticated if they left the bluetooth device transmitting. Pretty stupid if you ask me.

If it were me, I would have either:

1. Require a secret pin to be entered in order to activate the bluetooth. 2. Don't use bluetooth. For example the nRF51822 chip (e.g. in this module [1] allows you to implement your own radio protocols. You could make it impossible to detect - it could only respond when send a secret code of some sort.

[1] http://www.seeedstudio.com/depot/Seeed-Micro-BLE-Module-w-Co...