When you have a strong feeling that someone is watching you, they most likely are. Developed long ago as a primal survival skill, your brain has a ‘gaze detection’ system that fires an alert whenever it notices someone is staring directly at you or looking a few degrees to your side – so suddenly getting that ‘eerie feeling’ of being watched could in fact be a signal that your subconscious has just caught someone staring you down. Source Source 2 Source 3