Never heard of that theory or even thought about it. It's possible...
If aliens have been visiting earth for more than a century, who knows what technologies they possess.
We have hi-tech helicopters (choppers?) created to "cloak" itself from enemy radar. And image these obviously highly advanced species with their technologies, compared to them we may still be considered to be part of the "primitive" species category (except with toys) of the universe.
Anyway, it does sound more possible for these aliens to have an existing base here on earth. There are still many acres of virgin land in North America that are yet to be populated. However, recently reading an article on U-M homepage it demonstrated...*searching for exact topic, don't know where it went(wondering where I read this now

)*...
Well wherever that topic is, it spoke about an amateur film being taken of an alien spacecraft flying near their home...this even happened on several occasions and was first spotted by the father, who had seen this UFO behind a large hill, of this family. And I think seen by an RCMP/ranger (not sure if this is an American or Canadian report), who arrived at this family's area after hearing such a ridicules event.
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Aaah, here it is, it was actually just a post
British Columbia, Canada, Dec 26th 2003, 11:30pm, Submitted by HBCC UFO