I don't think there is sufficient evidence to say that an automated probe has not already visited our solar system. Let's consider two probe types Von Neumann and Bracewell
Von Neumann:
1) They arrived too early for us to have a chance of detecting them and they left little trace
2) One or more have arrived recently but they are discrete e.g. no active propulsion, passive sensors, tight beam communication, they only harvest a small amount of material for self-replication etc.
3) We just aren't looking hard enough, space is big and we have limited resources devoted to this pursuit
4) We've observed the activity of a probe but we mistook it for an anomaly or a natural occurrence
Bracewell:
1) Non-self replicating and arrived and left too early
2) Present but not fully autonomous and awaiting confirmation to begin contact (could be a long time waiting depending on distance from home)
3) Present, fully autonomous but we haven't met the criteria to begin contact
4) Present, fully autonomous, has attempted to contact us but we lack the ability to detect and understand their method of communication
5) Present, fully autonomous, has successfully contacted us but whoever was in contact has successfully convinced the probe to not make itself more widely known and has managed to keep their own communication with it a secret (probably the most unlikely scenario but still possible if the probe has protocols for dealing with local geopolitics)
If we're talking about colonization by the alien species itself there are still further explanations for silence:
1) All civilizations capable of reaching us have some equivalent of the "Prime Directive" where they do not forcibly colonize or contact primitive civilizations
2) No civilizations attempt to colonize the entire galaxy, perhaps they all focus on fully utilizing each solar system as they need it, maybe they eventually run out of needs that require expansion. The human population is trending towards replacement level birthrate within the next 100 years, if that is a common trend for all intelligent life then their population may simply stop expanding at some point and after a limited number of systems are colonized they may not have enough population to support further expansion. Maybe birthrates actually trend to 0 and lifespan approaches immortality.
3) Maybe colonizing planets is a dumb idea. Gravity wells are hard to get out of so once your civilization moves to space maybe it just stays in space. We are proposing generation ships as a main method of colonization but if the ship is good enough to live on for generations then why disembark? Why not just continue to live in space habitats and travel the galaxy continuously? Planets are dangerous and unpredictable with uncontrolled weather, volcanism, asteroid impacts, solar flares, etc. If you build a synthetic world you don't need to worry about that.
4) There might be other alternatives, upload to VR, conversion to another form of matter, time travel, alternate universes. These are pretty unlikely but so is the probability that we are the only life to have emerged in such a vast galaxy.
Von Neumann: 1) They arrived too early for us to have a chance of detecting them and they left little trace 2) One or more have arrived recently but they are discrete e.g. no active propulsion, passive sensors, tight beam communication, they only harvest a small amount of material for self-replication etc. 3) We just aren't looking hard enough, space is big and we have limited resources devoted to this pursuit 4) We've observed the activity of a probe but we mistook it for an anomaly or a natural occurrence
Bracewell: 1) Non-self replicating and arrived and left too early 2) Present but not fully autonomous and awaiting confirmation to begin contact (could be a long time waiting depending on distance from home) 3) Present, fully autonomous but we haven't met the criteria to begin contact 4) Present, fully autonomous, has attempted to contact us but we lack the ability to detect and understand their method of communication 5) Present, fully autonomous, has successfully contacted us but whoever was in contact has successfully convinced the probe to not make itself more widely known and has managed to keep their own communication with it a secret (probably the most unlikely scenario but still possible if the probe has protocols for dealing with local geopolitics)
If we're talking about colonization by the alien species itself there are still further explanations for silence:
1) All civilizations capable of reaching us have some equivalent of the "Prime Directive" where they do not forcibly colonize or contact primitive civilizations 2) No civilizations attempt to colonize the entire galaxy, perhaps they all focus on fully utilizing each solar system as they need it, maybe they eventually run out of needs that require expansion. The human population is trending towards replacement level birthrate within the next 100 years, if that is a common trend for all intelligent life then their population may simply stop expanding at some point and after a limited number of systems are colonized they may not have enough population to support further expansion. Maybe birthrates actually trend to 0 and lifespan approaches immortality. 3) Maybe colonizing planets is a dumb idea. Gravity wells are hard to get out of so once your civilization moves to space maybe it just stays in space. We are proposing generation ships as a main method of colonization but if the ship is good enough to live on for generations then why disembark? Why not just continue to live in space habitats and travel the galaxy continuously? Planets are dangerous and unpredictable with uncontrolled weather, volcanism, asteroid impacts, solar flares, etc. If you build a synthetic world you don't need to worry about that. 4) There might be other alternatives, upload to VR, conversion to another form of matter, time travel, alternate universes. These are pretty unlikely but so is the probability that we are the only life to have emerged in such a vast galaxy.