They knew with the lights out no one could really see into their apartment, but they could see out. They could watch people in the building opposite them doing dishes, watching TV, etc. It seemed so much more erotic to know that while the neighbors went innocently about their various daily lives, they were fucking. She loved being pressed to the window and fucked in the back. He'd pinch her nipples, or rub her clit, or spread her lips wide. The more lewd it was, the hotter it was that no one could see, but almost could. Darkness is the thinnest of curtains.