If there are technological aliens in our galaxy, then it is possible that their space junk could be part of the Interstellar material that makes it to our solar system. How much depends upon how many technological alien civilizations there are and how much junk is made from what they are doing.
It will be decades to centuries before we can go to another solar system or even very far beyond Pluto. We are able to take improving looks at what is in our solar system and what happens to drift into our solar system for evidence of Aliens.
A Lot of Interstellar Space Junk Seems Inevitable If Aliens are Making Spaceships or Probes
Space is huge so we and aliens will probably have to move at very high speeds like 20 to 99% of the speed of light if they want travel times to be as economically short as possible.
If they are using various kinds of spaceships and probes and they are moving at high speeds then parts could break off intentionally or unintentionally. The spaceships and probes would move at high speeds between solar systems almost all the time. They would not be able to stop and gather any part that broke off. Voyager 1 will run out of power and stop working in 2025. It will then be mainly inert and will be unable to communicate with Earth. Eventually it could run into something.
If Aliens were not making round trips or were not moving between two alien habitats then probes and spaceships will eventually wear out.
All fly-by missions end up with something that all becomes space junk.
The world has about 5 to 13 million tons of plastic waste every year in our rivers, lakes and oceans. This is tens of billions of pieces of man-made junk that mixes in with plants, fish and debris from nature. Our manmade ocean junk is mainly parts of fishing nets and fishing related material. Natural stuff ranges from bits of seaweed to icebergs. If you were sitting on a small island and look at what washed up on the beach it would be a lot more natural material than man-made. However, wait around long enough and there would occasionally be man-made junk.
Earth orbit is filled with space junk from our satellites. Earth orbit was relatively free of material and then we put up satellites and rockets that collided or shed parts. There might be more man-made space junk than natural meteorites in our heavily used earth orbits.
A lot of What We Detect In Space We Do Not Really See. Assumptions, Logic and Analysis Are Mixed in With Dots
We assume natural space rocks dominate the solar system and the universe. We can only see the big ones in detail. We detect small objects that we can barely see as dots. There is a lot we cannot see at all but we logically assume is there based upon what we can see. There is what we can see as dots. There is the far larger things we can see in some detail.
In Astronomy, there is a lot of analysis of the light of dots to tease out information. What is the color of the dot? How fast is it moving? What is the light curve like? All kinds of tricks to get information from dots. Some of it is based upon assumptions. If the light gets less bright and we assume the light is a star then probably something passed in front of it. We assume that what passed in front of the star was usually a planet. Then we think about the size if it was a planet based on how much less bright the star got. However, it could also have been a far closer dust cloud.
What Size Rocks Can We See Near Jupiter?
There are 79 known moons of Jupiter. In 1610, Galileo could see four moons as light smudges with his 30X telescope. By the time that the Voyager space probes reached Jupiter, around 1979, 13 moons had been discovered. Five through 13 were about 14 to 100 kilometers in size. This did not include Themisto (4 kilometers wide) which we kind of saw in 1975 but then lost track of until 2000. Voyager found three inner moons in 1979: Metis (2 kilometers wide); Adrastea (20 kilometers wide); and Thebe (100 kilometers wide).
1999-2003, 34 tiny moons, in weird orbits and averaging 3 km (1.9 mi) in diameter were found.
2004-2017, 17 more moons found.
2018, there was confirmation that ten more moons were found.
If Jupiter moon-sized 2-kilometer objects were out at the distance Jupiter from Earth but not right beside Jupiter then we would not find them.
A new telescope with an LSST abbreviated name will really begin operating in 2022. This will let us see what we expect will be hundreds of objects in the 1-kilometer size range at Jupiter distance.
Near Earth We Can See Smaller Rocks than the moons at Jupiter
We can see smaller rocks. We can only some 1-kilometer diameter near-earth asteroids and very few 100-meter sized asteroids. LSST should let us find 50,000 near earth asteroids in the 100-meter size class in its first 60 working days in 2022. This will be on top of the 18,000 mostly larger rocks we know about now. This is out of an estimated 3 million near earth asteroids.