Aisa is living in skypiea since she was born, knowing everyone's presence(could feel when Wiper is angry just by reading his aura). She should already have had "preexisting sense of awareness". Yet she didn't even feel Wiper, let alone others, who wasn't far away from the snake and she admitted herself that it(her observation) simply couldn't work
In Enel's case, it's different. His Haki is further enhanced by his Devil Fruit powers, making him virtually omniscient of the thoughts and actions of Skypiea. With the Goro Goro no Mi, Enel can pick up electromagnetic waves, allowing him to overhear conversations and everything around him: this has extended the range of his Mantra to cover roughly all of Skypiea. So regardless if he knows you or not, he can feel or even hear you. Notice that the moment Luffy left the snake, he directly noticed his presence
Sanji already knew how Kinemon's torso could feel since he already felt Kinemon's aura as well