Bluetooth beacon/sensor

Anyone knows any bluetooth beacon/sensor that can be used to emit bluetooth radio waves constantly so that other devices with bluetooth (such as mobiles) can detect them?

I don't even know if the beacons can be regular bluetooth devices or if they have to be different or special. Any models, prices, products that can do what I described?

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I don't think I understand. What would be the purpose of a device such as that?

The first thought that came to mind when I read this post is having the bluetooth act as a beacon. This would come in handy when looking for a missing person perhaps? However, this is probably not what the post is about, huh?