I have reservations concerning how this technology is going to work. Why Bluetooth ? For the first part, this could make the software vulnerable to exploit via “Blue Jacking”, and probably even worse if you used NFC. What would be far more accurate is to poll the nearest cell tower / mobile mast for a list of registered devices, and then see if any of those are in proximity to yourself. If you were in the same location, you’d be on the same tower, therefore, the results would be more accurate. Of course, there is a privacy issue here, and it would also mean that certain information would need to be redacted in order to respect that.
Another way of implementing this is to create a Wifi hotspot in shops etc that can provide information to the installed application when it in range. Joining the access point (obviously, not a Wifi pineapple or similar point, and it would need to be under the control of the owner). If others are already peered with the access point, then that information can be used to alert others in the nearby vicinity.