This is my belief that they stay out of our visual spectrum. they are there just we need infrared to see them.
One night I was star gazing and thinking about Ufo's. I kept thinking, just give me a sign, to confirm my beliefs.
Well all of a sudden, directly where I was staring they gave me a glimpse. It looked like number six in his photos. It was hard to describe, but it was big . It was an orb like thing, no flashing lights like an airplane. I was thankful for this glimpse and have not asked since, because they gave me my confirmation.
I believe they can power up and stay invisible if wanted.