As noted, given the size and age of the universe, and the relative rapidity at which dispersion of intelligent life can occur, evidence of alien colonization attempts might plausibly be discovered. A practical means to overcome the limited span of life of species and civilizations is artificial probes
Some theoretical exploration techniques such as the Von Neumann probe (a self-replicating device) could exhaustively explore a galaxy the size of the Milky Way in as little as half a million years, with comparatively little investment in materials and energy relative to the results. If even a single civilization in the Milky Way attempted this, such probes could spread throughout the entire galaxy. Evidence of such probes might be found in the solar system—perhaps in the asteroid belt where raw materials would be plentiful and easily accessed.
Another possibility for contact with an alien probe—one that would be trying to find human beings—is an alien Bracewell probe. Such a device would be an autonomous space probe whose purpose is to seek out and communicate with alien civilizations (as opposed to Von Neumann probes, which are usually described as purely exploratory). These were proposed as an alternative to carrying a slow speed-of-light dialogue between vastly distant neighbors.
Rather than contending with the long delays a radio dialogue would suffer, a probe housing an artificial intelligence would seek out an alien civilization to carry on a close range communication with the discovered civilization. The findings of such a probe would still have to be transmitted to the home civilization at light speed, but an information-gathering dialogue could be conducted in real time.
Since the 1950s, direct exploration has been carried out on a small fraction of the solar system and no evidence that it has ever been visited by alien colonists, or probes, has been uncovered. Detailed exploration of areas of the solar system where resources would be plentiful—such as the asteroids, the Kuiper belt, the Oort cloud and the planetary ring systems—may theoretically yet produce evidence of alien exploration, though these regions are vast and difficult to investigate.