Any active Bluetooth device, within range of your iPhone, will appear in the list of Bluetooth devices on your phone. This is basically no different than any nearby Wi-Fi networks appearing in the Wi-Fi list.
Just because the device appears, doesn't mean that it is paired with your phone. That requires user intervention to make that pairing successful.
To try to find out what device this is, on your iPhone, go to Settings > Bluetooth. Locate the device, and then, tap on the info icon. This should reveal information about the device.