In my opinion there are several possibilities for UFOs if they are indeed physical craft that are flying around.
1. They are aliens from another star system.
This is the most popular choice as the aliens could be from another star system and their motivation ranges from benevolent observation all the way to an insidious invasion agenda. There could be multiple species if this is indeed true.
2. They are actually US from the future.
This is one recent theory where in our distant future we come back in time to check out our own history and probe the occasional redneck. Some reasons are that they may have genetic issues caused by cloning and they are looking for “fresh” genetic material.
3. They are actually US but from the PAST.
This is one theory no one besides a few others have brought up. However, it has taken only 6 thousand years to go from hunter gather type society to full blow technological species. Humans have been relatively unchanged for about 250,000 - 100,000 years. Neadertals had bigger brains and are even older. It only takes about 10,000 years for most evidence of a modern society to be worn away by nature, even less if glaciers run over it. We have had several ice ages in the time period that humans were unchanged. We could have had a civilisation that was formed 100,000 years ago and they came up with tech and left this planet and their descendants have come back to check on thiose left behind.
4. They are from Earth but not US.
This is the hypothesis that intelligent beings came about from a dinosaur line and they are living underground and curious about this new seemingly intelligent species known as humans.
5. They could be a combination of the ones above....
Personally I think #3 is more likely given the relative impossibility of interstellar flight.
Number four was the theme of a recent Dr Who double episode.
There are artifact objects in our solar system which have not been put where they are by our current civilization. People poo poo that reality but I happen to know a person who worked for NASA airbrushing out the objects when photographed during NASA missions. That lends credence to your number three posit.
I suspect those objects remain as evidence that a previous Earth sourced civilization got off the planet in some distant past epoch.
I cannot reason out why an extra-solar civilization would leave debris behind, having gone tot he great effort to fabricate their materials. But if a civilization sourced in Earth's past became suddenly destroyed then the remains of that civilization could survive as artifacts on the Moon, on Phobos, and on other solar objects.
#3 - they/we fled Earth (due to some catastrophe?) and have been living elsewhere in the solar system? Perhaps in hollowed out planetoids? Visiting the recovering planet and Waiting for the right time/conditions to return to stay?