My take on all this is the following:
1 - as to how a being might hide in plain sight...
Movement to a place where we aren't is faily easy, as Umbran pointed out, wwe spend far too much time looking 'way' out there and not enough looking 'just' out there; as to the possibility of radio or other 'ray' emitting recievers picking them up? - Hey we have a bomber that has the radar signature of a parakeet, imagine what stealth would look like in space with all that background noise?
2 - As to the How...
This would be the big thing, according to physics (and since it is the Law of the Universe I will stand only on these principals) the Theory of Relativity states that faster than light travel is impossible for being of matter, if aliens are indeed gray skinned bug-eyed runts, then that means they have some form of matter and therefore cannot travel faster than light. So then we have to think of alternatives, such as the wormhole theory, meaning that the aliens have been traveling through folds in space by either using existing ones (the closest black hole is still some distance away if you believe that they are actually portals) or by slicing into space and time and creating them artificially. If this is possible the savings would be awesome, the science incredible and the test runs I'll leave to someone else...
3 - Why?
After 60 years of stealth like probes, anal research and sex with Elvis, Jimi Hendrix, Janis Joplin and Jim Morrison, you would think that first contact would be on the docket... but how about this, what if like our own world, there are several nations and each is competing to seek out allies in an intergalactic confrontation? Why assume that all little gray men are from the same one world community?
4 - My personal conclusions - Becuase it makes us feel better. If an alien culture can eventually overcome its differences and unite on a global scale to achieve interstellar travel and breaks the laws of physics in the process, then we should, theoretically, be able to do likewise. I hate to go 'Star Trek' on everyone but in the original series it was only AFTER the world united in one government that real space travel was achieved. A world economy focused on singular goals with nearly unlimited resources made that 'trek' possible.
Let's face it, who doesn't want a united world order where peace reigns supreme...oh yeah, a lot of people. I personally don't feel that there is any alien probing going on at least not on the scale that has been reported over the years. Were I an alien culture, I would be monitoring our radio, television and satellite communications from a hidden base waiting for progress to peace if I were a culture of that type or waiting for a conquering hero if that were my bent, either way, I wouldn't be continually sending in scout teams to take colonoscopy readings of the locals, unless of course I were havesting something in the bodies of those humans for my physiological advancement (yep - lab rats).
Applying human logic to a non-human problem is as good as we are going to get, and once an incident happens mass hysteria gives rise to other 'non-events' with similar circumstances giving more credence to the possibility. qv Salem Witch Trials. Even on a low key issue such as this, and in the scheme of things it is low key, being abducted is a way to be a 'celebrity' at least for a little while. I personally don't think there is another intelligent matter formed life force in the universe. I know, I know, but what about the "billions and billions of galaxies?" Frankly, Calr Sagan was a pompus arogant imbecile who thought he was smarter than he was. Yes there are infinite solar systems, but if the accidental creation theory is correct then the likelyhood of a similar event happening elsewhere are (pardon the pun) astronomical. And if the divine creation theory is espoused then it is even more likely that there is nothing else out there, as all things are purpose driven and therefore a
chance meeting with another creation would be counter productive.
I'm sure there are tons of holes in my post - its late, I'm exhausted and the little grey man giving me an anal probe is not warming his instruments beforehand so I am, quite understandably, distracted.
