Seems to me they would have noticed a bunch of guys in the field "hoaxing" the formations...
In my youthful days of sneaking up to exactly that type of installation with make believe C-4 in my alice pack with my buddies (on Uncle Sam's payroll) I found out that many of them are heavily automated, and you are hard pressed to find any living human who is not buried deep in an airconditioned windowless cinderblock building. The "security" consists of a retired cop who sleeps in a corner and goes out to take a leak every hour or so, and gets about 100 feet from his soft chair by the TV set.
You could light that field on fire after marching through it with a brass band and not get noticed.
I speak from personal experience of skulking through many farmers fields at midnight in Europe and America with 4 to 8 guys loaded with packs and guns. I worried MUCH more about unknowingly crossing into a bull's pasture than waking up the farmers, or the "security" (hahahaha!) at the target.
Sneaking up with camoflauge and night vision goggles isn't quite the same as setting up the means to create the images below, which would be more than a simple matter in the daytime even if given quite some time to do..
That dot matrix face would be just a bit difficult to create I think.
Then when you look at the response to the "Arecibo message", well, it's more than possible that it wasn't created by guys with C-4 either..
I lived near a cattle farm that bordered my "burb" or subdivision as they were called back then and they usually had some Brangas and Angus and Brahmin bulls penned off in 5 -10 acre lots. We would of course go and taunt the bulls. Funny thing about bulls...one day they will act like you are not there. The next day they will try to crush you into the fence. I can imagine a rowdy bull being rather aggravated by a slinking group of you and your pals easing thru his domain at night. That would have made a funny video. You and your team scampering across a field with a bull in pursuit. I suppose you guys were encouraged to avoid local livestock casualties if at all possible..lol. I bet your average farmer would be rather suprised to find his bull the next morning shot to hell by fire bursts. That combined with alien crop circles could drive one over the edge.