Justifications for this belief include the way some UFOs appear to rapidly accelerate and decelerate, the way they appear to travel at tremendous speeds, or their capacity to blink in and out of sight, as if hopping between dimensions.
The fact that UFOs are more often than not completely silent is also commonly mentioned as proof that they operate via a technology far beyond our understanding.
I'm not so sure this is as obvious as it is commonly taken to be.
Projected and/or holographic images also can be made to seem to blink in and out, or 'fly' silently overhead. Although sometimes radar evidence would seem to indicate a solid object, not a projected image, if someone was bent on deception there are ways to 'fool' radar.
In fact, the US government has developed (and is probably developing, as we speak) some of these methods. One of the best examples is the stealth bomber, which was mistaken for a UFO often before its existence was disclosed.
I am not saying that no UFOs are a true mystery or that the US government is responsible for all of them by way of weapons development. I'm just saying that given the propensity for deception that has come to be a major part of the phenomena, we should be difficult about projecting our own ideas onto something that might not be what it appears to be.
Somehow, people who want to understand the phenomena have to disentangle themselves from the ET hypothesis and stop projecting ideas that fit that hypothesis onto the UFOs themselves.
I personally think that pushing human beings toward a greater tolerance of paradox and uncertainty is at least part of what is going on.
But I should probably not get too invested in that either.