It seems that Google has now rolled out a new Google Maps feature, searching for a city, a country or a ZIP Code will return a map with the contour of that city or country highlighted. Google has been testing this feature with some users for a few weeks at least, it was spotted in December by some.
The move makes sense, when the search is for a big place, not a specific location, having the area highlighted provides better context and also serves to better pop out the actual place you're looking for.
At the default zoom level, a pink border will be drawn around the city or region. Zoom out enough and eventually the entire region is shaded pink. Zooming in also shows the pink border, but it becomes a bit more transparent and smaller.