Non-Extraterrestrial UFOs

Interesting NASA survey described in the Atlantic about the limits of science and technology to detect past civilizations.

Tensions

Difficult to reconcile:

1. Our current narratives about technological advancement, and human biological superiority (the idea that humanity represents the endpoint and apex of evolutionary development).
2. UFOs declassified by the military.
3. SETI (and the general lack of scientific evidence despite immense funding and effort)
4. The Fermi Paradox

Part of the above can be explained:

1. Deconstructing assumptions about communication and communication technologies.
2. Deconstructing assumptions about space travel.
3. Lack of sophisticated sensor technology (advanced radar).

On 1, we assume that communication is human-like using current or near-current technologies. Anything outside that range is something we would miss.

On 2, we assume that space travel is the end objective, not simulation or miniaturization.

I agree that it's doubtful dinosaurs ever evolved to human-like technology levels. But, let’s explore that interesting notion (that anything older than a couple of million years would be almost impossible to detect - fossils, tools, and even the hallmarks of advanced societies like skyscrapers will decay and be buried deep in the crust after a few million years).

Considering Past Civilizations

1. The last dinosaurs were around about 65 million years ago. Far greater than the 2 million years of geological and paleontological history we can currently confirm with great accuracy (save for the random chance of finding a specific fossil - there are only 2,000 or so complete recovered fossils apparently out of the estimated hundreds of millions of large-life forms that have likely existed on Earth).
2. Humans, as we are today, have been around for about 300,000 with about 10,000 estimated years of civilization (writing, agriculture, fire, cities, etc.).
3. Dinosaurs were around for > 165 million years and the total snapshot of time for complex life on Earth is probably > 300 million years.

So the math says that there’s plenty of time for other advanced specifies to have emerged, died off, and/or remain hidden from our current technological survey systems (radar, LIDAR).

Biological Characteristics

I think that the most plausible features of such a candidate species would be (as many of) the following (as possible):

1. Very small - lower energy needs. Less food. More populous per square mile.
2. High brain-to-body ratio. Smart. The body is efficient with energy and neurons.
3. Massively reproducing - lots of babies. High survivability rate. High gene mutation rate.
4. Having arms or some equivalent way to manipulate the environment.
5. Fast - probably four-legged. Low center of mass. Most animals aren’t bipedal.
6. Omnivores - not reliant on specific niches of the biome for sustenance.
7. Underground or flying - difficult to find by predators. Shielded from the weather by migration or earth.
8. Terrestrial - unlikely they would be sea creatures at the beginning since fire and combustion seem to be essential for most advanced technological processes.
9. Aggressively bioengineered - would have aggressively modified their gene structure after achieving that level of knowledge - reducing their size even further, increasing their health/immunities, eliminating aging, and allowing them to live underwater or in space.
10. Some forms of persistent communication - writing, pheromones, etc. (writing as we know it is almost entirely dependent on our anatomical features: hands, bilaterality, two eyes, the primacy of the visual system, and so on). Something that allows sound and meaning to be made less transient so that ideas can be shared that outlast a single individual's lifetime, moment, or singular events (disasters, discovery, conflict, etc.).

Space Travel

The above article resituates the locus of our speculation upon the terrestrial and earthly. What about space travel in such a scenario? A smattering of concerns:

1. Space travel is very expensive - in terms of the rarity of resources, technology development, knowledge accretion, etc.
2. Assuming > 2 million years since the birth of such a hypothetical species, they’d have millions of years to cross into space and between solar systems.
3. Many planetary bodies are well within the range of such time-boxed travel itineraries. For instance, Alpha Centauri is 4.37 light years away.

In the discussion of space travel, I think we tend to overlook the usefulness of miniaturization. Star Wars depicts massive fleets of gigantic battle cruisers, X-Com's Earth is invaded by city-size motherships, Independence Day is the same, etc. While those are awesome franchises, scientifically-speaking miniaturization has rarely been considered in Science Fiction (Three-Body Problem being the only franchise I'm aware of that employs that directly as a narrative conceit).

Miniaturization is advantageous for many reasons: it makes a spacecraft difficult to detect (and attack), a spacecraft would probably have lower energy needs, and one is able to have a higher density of spacecraft per unit of space improving survivability and redundancy as one spreads across the infinite expanse (of spacetime).

I think that miniaturization would also be an option for such a species. Provided they could achieve miniaturization technologies, I'd think they'd also still have the ability to produce large spaceships, xenomorphs, etc. (For humanity, miniaturization is likely to come in at the tail end of technological development. Even XNA - yes, xeno nucleic acid, has been solved before the so-called "fat fingers problem".)

The presence of miniaturization in some past (yet, futuristic) animal societies might also help explain the absence of various artifacts being discovered.

Shadowy Human Institutions: Drug Cartels

To create advanced drone technology (and manufacture it) would seem to require fairly strong infrastructure and scientific capabilities that are "off the grid" (so to speak).

I can think of no other set of current/extant organizations/institutions with those capabilities save for the Drug Cartels who have near-complete dominance over several countries in Central America now.

They have far-ranging science, logistics, and infrastructure capacities that extend across continents. They can manufacture advanced and synthetic chemicals with relative ease. Is it possible they dabbled in aerospace and achieved certain breakthroughs? (They certainly don't disclose their chemistry research with the wider scientific community.)