Well that was a suprise, I walk that area pretty frequently! As far as I can tell they're some of the footpaths around Whiteways, what map do they appear on? Do you cycle/walk them? In which case they're probably the Garmin trip log lines.
They appear on the GC route map on the Activity screen. My route was somewhere else entirely different but mine had a black line along a street, around a corner for no apparent reason so I started to scroll around and found those.
Not sure what a trip log line is, but the one on the street near me makes no sense as its hardly scenic and is less than a mile long, and certainly nothing associated with my activity.
They're cycle routes, a recent addition to Google maps which have actually been visible via Connect for a while. Fire up Google maps, go to the same area in map view, click on the little box which says Traffic with an arrowhead at the bottom, click on Bicycling and those lines will reappear, and you'll see a colour code under Bicycling for the three different things it shows - trails (as seen here), dedicated lanes, & bike-friendly roads.
