Unless I'm missing something, and I may be, this is relying on the geotagging information you include in tweets, something you can control. I'm not sure I see much advantage in having your location broadcast every time you tweet.
Many users don't know that something is on by default, and if the default is to geotag, people will nearly always forget to disable the geotagging.
You or I might know what it means when the 'place' icon is highlighted during a tweet. Non technical users might not understand the implication of thier actions.