"What if to the 'aliens' this is their home? What if we unknowingly are sharing the earth"

Chances are we already are: Growth and replication of red rain cells at 121 oC and their red fluorescence.

The thing is, we assume aliens are going to be like us. More than likely we're not even going to be able to recognise them if they did land on the White House lawn tomorrow morning as the will be so radically alien in form and culture we would struggle to tell which way up they were.

An alien could be a sentient gas cloud, many light years across that takes years to form a thought. We could fly right through it in a spaceship and not even realise it's existence. They might be silicone-based and need to breathe liquid hydrocarbons and eat olivine (shades of the burrowing Horta!) etc.

They might be able to manipulate the physical universe in ways we would be unable to distinguish from magic (LH's time travel might be a breeze to them).

As for greys and flying saucers - these are cultural interpretations of misunderstood or misidentified natural phenomena or conventional aircraft, even if they are secret government projects. In the middle ages they were wheels and stars, now they're spaceships; in 500 years they'll be something else.

But here's a thing (a true one too). This summer my wife and I travelled across the Atlantic and spend three weeks in the American West. The first week was spent in the field digging for dinosaurs in the badlands of the Hell Creek formation of North Dakota and Montana. On our last day in the field our two-car convoy was headed north on a highway close to the ND border when a flash of movement on the far horizon caught my eye. I watched a sleek object shoot up at an incredible speed and on a trajectory that would be impossible for a fixed wing aircraft. I pointed it out to the other four people in the car and as one of the chaps commented, it "was haulin' ". It might have been a missile or something but it was really shifting, leaving a sharp trail through the clouds but no trail in the blue sky. I can honestly say I've never seen anything like it in my life, and I've lived near airports all my life.