This took me a month or two to spot and track down, so I am posting here for the benefit of others.There was nothing about this on Google.
I am an iPhone developer, and I eventually diagnosed this by setting "Find My Friends" to geofence my real location and send a message when it spotted me leave. The second I started running an app on Xcode (the Apple software used for developing iPhone and iPad apps) it pinged instantly.
There is a setting in XCode that sets the GPS to a chosen dummy location whenever you run an app. It can be found by selecting Product>Scheme>Edit Scheme from the top menu, then select "Run appname" on the left and "Options" at the top. The one you want is "Core Location: Allow Location Simulation."
Just uncheck that box. Problem solved.