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Strava.

Fitness app Strava lights up staff at military bases - https://www.bbc.co.uk/news/technology-42853072.amp (Jan 2018).

What did I do now?

By the way, I remember a similar case in Russia. A telco company published a heat map created from GSM location data that reflected where in the city yound people spend their time. One of the "hot" spots on the map was in a remote forest area with nobody living around. Most probably it was a military object.

"secret running track" feels like a joke, but the point it illustrates is clear - the massive uncontrolled dispersion of location data.

There was also a case of soldiers using flash-card apps to learn about the nuclear bases they were stationed at, revealing various facts.

https://www.bellingcat.com/news/2021/05/28/us-soldiers-expos...

This shows that people generally don't understand client-server architecture and think that the app stores data on their phone and not somewhere in cloud.