Alright, Arnold aside. What about other observers, including those trained to differentiate Russian bombers from flocks of birds and other aerial phenomena? Those given warnings via JANAP (Joint Army Navy Air Publication) about the situation which actually existed? Arnold is not a one off event.
It depends?
See an observer will tend to 'see' what they are trained and expecting to see and when they don't they will then try and interpret what they see AS something they are trained and are expecting to see. And quite often during that process they will interpret what they see, especially if they haven't managed to fit the observed object into a frame of reference, they will then ascribe aspects and attributes to the object that in fact don't match the object but do rationalize a pattern in their own mind.
Specifically this can be a problem when someone is told to look for something or "watch" for something from an official source and they then go out looking for what they were told to look for. It's not a Russian bomber so it must be something else, what else is the question and the thing is it might in fact BE a flock of birds, just in a way the observer has never seen them before.
Example:
Now I'm not a trained observer but have both a fascination and interest in aircraft and enough military training to be able to rapidly identify and categorize aircraft and things that fly.
Getting out of my car near the local college campus (I had a class to attend) in clear, broad daylight near a semi-busy street. Something causes me to look up at the mountains in the background of the collage and I see something floating against them. Air is still with no breeze, and the air is still and cool with some frost still on the ground, but not on the streets or sidewalks even though it's moving towards mid-afternoon. It appears to be a balloon as it is a large white envelope but it appears to be moving far to fast for a balloon given its apparent size and relative motion against those backdrop mountains. It is maintaining level flight and speed but it seems to square for a standard balloon envelope and there is no basket or structure under it. Runaway balloon maybe? Then note that the envelope has two hanging segments, maybe three and is slowly rotating. Still doesn't explain the rapid speed it is moving and I can make out some red almost translucent markings on the side...
SNAP
It's a plastic shopping bag floating along somewhere between the sidewalk and the campus, possibly lifted by hot air and travelling by being pushed by an above street breeze, (I can see the upper branch leaves now) My original scale was false as was my speed estimation and the bag is slowly dropping in altitude but for about five minutes there, (actually more like two I checked by car clock) it was a real UFO.
Now imagine that "snap" never happens and the observations continue trying to 'explain' and then justify what and how you are seeing the object. Now extend that to other observational media like radar images and video or computer storage. The storage media doesn't "lie" but the data has to be interpreted by humans so often is, and not always correctly
en.wikipedia.org
Of course this doesn't "explain" everything and it shouldn't as there are real phenomenon out there that need to be explored and studied. But "unexplained" usually simply means "we can't explain it at the moment" not that its aliens or flying saucers per-se
Randy