Nasa has what? a shield device. What does it shield, heat, electronic energies, where do thoes energies go?
Log into the site atomic rocket, I've suppplied the link.
The site author is an engineer and has looked very hard at imaginary devicies that pretend to hide energy.
What you do not understand about space is that it is very very very cold. any heat stands out like, "As of 2013, the Voyager 1 space probe is about
18 billion kilometers away from Terra and its radio signal is a pathetic 20 watts
(or about as dim as the light bulb in your refrigerator). But as faint as it is, the
Green Bank telescope can pick it out from the background noise in one second flat."
Let us assume that there are aliens with the technology to get here and stay in our solar system without any of our instruments detecting them.
That means, they cannot be seen by eyes, radar, heat sensitive equipement, radio recievers. etc.
We would not know that they were here unless they made themselves known to us.
As we have been broadcasting radio and tv into space for a 100 years and as we cannot detect them, they could easily pick up a few people learn our language etc, dispose of those people and we would not know.
Why should any aliens make such a cock up of making themselves know to us?
Two answers. 1/ there aren't any.
2/ they have not revealed themselves to us, we have no idea they are or have been here because we have nothing they want.
Either way no aliens.