The Finnish company Polar - which producers fitness tracking watches and hardware - uses its main app to track activity, and a joint investigation from De Correspondent and Bellingcat has shed some light on how it has been used.
The app's tracking map Explore has, according to the investigation, exposed the home addresses of thousands of users, including secret agents and soldiers.
Part of the reason for this is that users will often turn the trackers on or off when you're almost home, which can reveal where they live.
As reported by Business Insider, the investigation has focused on 200 sensitive locations and found 6,460 individuals acoss 69 nationalities through site scraping techniaues.
The two organisations used it to find an area such as a military base, used an exercise published there and checked where the same user profile has been.
They also discovered the names and addrseses of people working for multiple intelligence agencies, including the US Secret Services, NSA and MI6 - with some of them using their real names.
In a statement after it suspended the Explore map, Polar said: "It is important to understand that Polar has not leaked any data, and there has been no breach of private data.
"Currently the vast majority of Polar customers maintain the default private profiles and private sessions data settings, and are not affected in any way by this case."