Knock Three-One-Two by Fredric Brown

He stood outside a living room window of the little three-room cottage looking in, watching her. By moving from one side of the window to the other he could see almost all of the room, and she, even if she looked toward the window, would not be able to see him. There was a net curtain inside the window. From the outer darkness he could see through it clearly into the lighted room, but from where she sat the curtain would be opaque. He could -- except for his Need and his desperate impatience -- stand here as long as he wished to make his plans and calculate his chances.

He thought they were good. The cottage was on the outskirts of town, in a neighborhood not very built up as yet. There were only a few houses in the block.