As mentioned, aliens can lurk nearby while invisible. In physics terminology this happens when they have not yet fully rotated themselves into our dimension, their quantum wave functions have not fully collapsed into our reality, or their quantum phases are not fully locked into our own. In occult terminology, this happens when they manifest within the etheric plane without condensing fully into the physical. The etheric is a transitionary boundary layer between the physical and nonphysical, functioning as an invisible substratum to the physical world but containing energies, beings, and constructs that influence the physical at the quantum level. Think of it as the backstage to our reality, the area behind the curtain of our limited five-sense perception.
In this invisible or etheric state, aliens can monitor targets without normally being noticed. By getting close to their targets, they can more effectively project strong thoughts into a person’s subconscious, giving him impulses, ideas, or inspirations that he will take as his own and act upon unless he questions their origins and rationality.
Greys in an etheric state are known to attach themselves to a person like an invisible parasite, subtly influencing the person’s thoughts, feelings, and behaviors even as he goes about his day. Aliens who require lifeforce energy for sustenance can also lurk in proximity to highly emotional, sexually charged, or distressing human activities to harvest the energy released.
Targets may also receive etheric implants, etheric gadgetry attached to them externally, and etheric devices or observation booths installed into their homes for monitoring and influencing purposes. This capability can be scaled up to allow alien bases to coexist on top of physical human military bases or airports, for example, dimensionally shifted and thus overlapping, seemingly occupying the same three dimensional space.