The technique used to listen in on a conversation using the window is an active technique (an IR laser is emitted towards the window to get the bounce), so it can be detected, and requires LOS to the window + to the reflected path if you're not hitting the window completely perpendicularly. For example you can't hit a window a floor above from ground level, and architectural treatments that result in angled windows can reflect light away in directions that are more difficult to observe.