It would be easy to hide such a vehicle. Just do your final deceleration burn in the outer solar system. The gravity of Jupiter or Saturn would be useful for an interstellar capture maneuver anyway. At that distance we'd be very unlikely to see anything, especially if it happened in 1940.
The little things sighted as "UFOs" would not be interstellar craft. They would be probes or local runabouts. The interstellar transport might resemble the craft depicted in the film Avatar (which was actually a realistic design), or maybe a hollowed out meteor. The latter would look natural when not executing a major engine burn.
As far as hiding: if they knew they were here to observe an intelligence with conceptual thinking ability and language (even one much less intelligent than they) they would know any overt contact would radically and permanently alter the subjects behavior.
This is all incredibly speculative. I'm just saying that it can't be ruled out by easy superficial arguments. It can't really be ruled out at all. We just don't have enough evidence to rule it in either.
Look up project Orion. How hard would it be to hide its deceleration from the system it’s entering. Any successful interstellar ship has to emit at least that much energy, if not far more.
In Zubrin's book Entering Space he says in general you should use the radiation pressure of the destination star or the solar wind to decelerate, and I believe published the work to demonstrate its feasibility.
The little things sighted as "UFOs" would not be interstellar craft. They would be probes or local runabouts. The interstellar transport might resemble the craft depicted in the film Avatar (which was actually a realistic design), or maybe a hollowed out meteor. The latter would look natural when not executing a major engine burn.
As far as hiding: if they knew they were here to observe an intelligence with conceptual thinking ability and language (even one much less intelligent than they) they would know any overt contact would radically and permanently alter the subjects behavior.
This is all incredibly speculative. I'm just saying that it can't be ruled out by easy superficial arguments. It can't really be ruled out at all. We just don't have enough evidence to rule it in either.