Maybe they are normally fully invisible, but even alien spaceships can have malfunctions every now and then. Perhaps they didn't realize the cloak system was off.

It’s weird, given the premise of seeing something alien, that you would assume properties of their propulsion or which emmissions they are making.

Sure, if the thing is some new earth tech or natural phenomena there are plenty of assumptions you can make. But in the face of something else?

The point is that you can make any number of stories about aliens trying to explain the phenomenon, and there would be some that would be way more believable then some object breaking pretty well established known physical laws with the only explanation that "aliens have ADVANCED SCIENCE"

For example, Aliens living among us for a while, undetected, and having the ability to place false memories in peoples heads because they mapped out how human brain works down to the neuron level (thanks to their massive compute clusters) is way more believable.

Why does it have to be off world tech? Take a smartphone back enough years and people are going to be similarly blown away.

Also you act like the science and the laws are settled but we all know science doesn't work like that. New discoveries happen all the time its that for the laws of physics those changes don't happen often.

What laws of physics revelations will the next Newton or Einstein offer us?

For a smartphone to be fundamentally unexplainable by relevant experts one would have to go back more than 100-200 years, I think. The first trans Atlantic radio transmission was in 1902, for example. Though I guess even the experts of the day might struggle to believe that the device is practically realizable, since it probably would defy many a conception of what is doable (if not theorically possible). There are some things now that seem impossible, which in 200 years might be commonplace. Idk, maybe residential grade or even portable nuclear power.

> There are some things now that seem impossible, which in 200 years might be commonplace.

Like flying very fast without producing a shockwave and only leaking some residual light and heat?

Man how awesome would a home reactor be.

The most plausible explanation that they could be fully invisible but chose not to be I've heard so far is from Robin Hanson, but there are several parts and that alone lowers its probability. Still, fun to think about, and also addresses things like how they could be here/find us at all, why are they here, what do they want to achieve. I'll risk a summary rather than just linking out... First they would be pan-spermia siblings from our stellar nursery, so they evolved in the same galaxy and could find their stellar siblings among all other stars. They're many years more advanced than us but for some reason they have a strong norm against expanding and rearranging our galaxy and beyond to their desires. We're still around so they don't want to wipe us out without reason, but we can guess that if we get ideas and capabilities of becoming grabby, they're here to nip it in the bud. Being sort of impressive but also mysterious can eventually let us know they are there without revealing too much since we could easily collectively find something out about them to hate and think of them as enemies. If we admire them instead we're more likely to try emulation including adopting a norm against spreading out to the stars as fast as we can before even more matter becomes unreachable or some other aliens from farther away galaxies arrive here in several hundred million years.

One benefit of this explanation is it predicts we'll see more of these strange phenomena even as our technology gets better and the possibility of simply being mistaken is driven way further down. If we stop seeing them suspiciously around the same time as big advances in sensor and sensor integration tech, all the more reason to bet on old observations being mistaken/hoaxes/conspiracies than actually aliens.

To your last point of seeing them less with more advanced sensors, wouldn’t you be able to say otherwise as well? Given that we have the higher capability to detect it would behoove a sufficiently advanced intelligence to stray away from that region. So I don’t think you can conclude with higher probability that it implies a hoax.

> If it didn't have a physical presence, like it was folding space or something and traveling in a bubble, then it seems very, very strange that an advanced civilization can fold space to effectively travel without any impact on the world, but let the EM radiation that is light/heat leak out.

I'm not sure why you're speculating on unknowable alien motives. The only thing that matters is agreeing on whether there is something unusual here that requires an explanation. If it turns out to be aliens, then you can speculate on their motives.