Interesting idea, but blue tooth has a limited range (~100 feet) and doesn't automatically pair with other devices, so the reported 'last known location' is likely to be where you left your bike and took your smart phone with you.

If a large enough network of these devices existed (and the users allowed their smartphones to forward data from the devices) they could serve the marketing purpose; however, large enough means encountering them regularly (likely need millions of them if not billions)... to cover a country and even an urban area would require enormous numbers of folks to not only purchase the device but also agree to data sharing for such a system to work. A minimum of 175 per square mile to give complete coverage that would provide a geographic location within a hundred feet... And a hundred feet in an urban area isn't particularly useful...