Stryder, I think you have a valid point regards stimuli vs reaction obsevation.
The thing is we can't quantify these CC's fully, except to say that they exist and that we call them what we call them.
Regardless of all the speculation and unknown factors we can not lay claim to a knowledge that is founded.
It is possibly a hoax, this being most probable.
It is possible that it be ET constructed but this is improbable.
It is as Stryder suggest possible that it may be a way of preparing people for a possible alien encounter....but not probable.
It is impossible for it to be constructed by natural means.
Even if you found evidence that suggested a lawn mower that would not exclude the possibility that an alien with a lawn mower did the job.
However I would say this to be improbable, but certainly I can not say it to be impossible for this would be a flaw in reasoning.
If we think that something is probable then we have to justify that probability some how.