Had the same type of upper surface wing ice 2day in CPH. It was over a smaller area, but it was upper wing ice. The flap and wing surface above the spar can maintain cold soak causing ice in these regions causing ice where the dew point temp is above temp of surfaces.
Answer for us was de ice. Tactile test sorts the problem of de ice or not de ice. Wouldn't judge on the the instance in question becouse I was not there.
I think descent profile on the inbound sector would have an effect also. Continuous descent at idle down to the marker will reduce the chance for the airframe to warm in the lower levels from -60deg in the cruise.
Deep and Fast
Only place I like ice is in a Gin and Tonic
