Firstly, big fat warning: this is not secure.
Bluetooth devices have MAC addresses in a similar vein to 802.11X wireless cards and 802.3 ethernet cards. You could periodically scan for the presence of a known MAC address. There are nominally 2^48 (281474976710656) possible such addresses but as with ethernet cards the first 24 bits will be reserved for a manufacturer, so if an attacker has observed your phone, then there are only 16777216 possible combinations for a known manufacturer. Given that it is possible to have the same number of manufacturers (16777216) and given there probably aren't that many, the attacker only needs an assigned list of known manufacturer codes to greatly reduce a brute force.
If your device has ever been bluetooth-enabled near an attacker aware that you are depending on the MAC address, then chances are high this information has again been compromised. Any OTA transmission will reveal your MAC, much as is the case for wireless access points, the only difference being the level of proximity to you that would be needed. Anyone who has ever paired with you will also have that address.
I don't honestly believe such a system adds much security in and of itself. Blunders is correct, you need additional secure link information. But if you're happy with just a number and don't care that it's about as secure as leaving your laptop in a pub with a post-it on it saying "steal me", this should be fine.
Here's an article that backs me up.