Not unflipped normal but some faces are unusually brighter? Why are these faces brighter than the rest? These are not flipped normal. After creating the window with edge loops, I deleted some edges and joined distant vertices to make face, thinking that doing so will reduce tris count. I shall use it in Unity.
I figured it out. I had left some faces detached from the same-object mesh. How could I do that!
All I had do to was to merge vertices of the loose faces with the vertices in respected positions. The edges of the  loose faces were positioned just beside other edges of the larger mesh. So visually, there were two edges side by side. Merging vertices solved the problem.