There was always something lurking in the shadows of Elysium. It was impossible to see or say exactly what it was, but you felt it perfectly. Whether it's a specific rustle of leaves or an unusually loud splash of Lethe going far beyond the fence bars.

Nothing changed except these fleeting sensations of someone else's presence.

When you notice the shadow, you realize that you are not so crazy. The shadow turns with a flourish, "Did I disturb you?"