Typically, if Google Maps is closed while in use, there will be a blue bar indicating that Google Maps is still in use. Recently this bar has stopped appearing for me.
Why might the location bar have stopped showing up?
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According to Techcrunch, iOS 11 will show the blue location bar when an app is using your location regardless of your privacy settings.
In iOS 10, if an app was set to use location “Only While Using the App”, and you pressed the Home button, a blue bar would appear. In iOS 11, the blue bar functionality has been expanded to those apps that are set to “Always,” too.
That means the blue status bar displays when those apps are using the continuous background location service – sort of like how the green bar shows up when a phone call is active, but you’ve exited from the Phone app to go to your home screen.
Check Settings → Privacy → Location Services → Google Maps and make sure location access for Google Maps is set to "While using the app". The blue bar won't show up if it's set to "Always".