In my opinion this is just tabloid sensationalism, a good story but completely irrelevant.
As how many RAF stations are an unknown secret? Also there's nothing to stop "the bad guys" fro observing anyone leaving the camp gates, even without a fitness tracker. The recent case at Marham is an example.
Or the "bad people" could simply sit outside the nearest pub on a Friday night.
From my own experiences that app has more positives than negatives. It's a great diary to log your runs and show you just how much or little you've done. You can even set yourself goals or targets and compare past runs to see improvements.
I would be more concerned about those Airmen who do absolutely no cardio fitness and struggle to pass their annual fitness test.