When someone is standing outside the window, it's possible to see the actual person blocking the window, but it also seems as if someone is sneaking around right behind them. Covering the outside of the window with a tarp effectively blocks the light completely, which stops the effect (though if left there, it will be gone by morning). The same would happen with the window being covered from inside. Nothing happens with the eye app at first, though the eye does glow on the window when drawn and viewed with the eye app.