We may not give a quantitative answer, but to me the odds seems low. On the one hand interstellar voyages are to my knowledge possible - and are in fact ongoing (Voyager and Pioneer and some others). On the other hand the duration makes them seem rather irrellevant,, and in addition it seems we do not know were they may end, even if they should exist for millions or billions years.
it may not be beyond imagination that this solar system, or even earth has been "visited" - or permanently "occupied" -by such things. But to find one if they should not have perished may make to find the famous needle in the haystack seems easy. And how much knowledge could we get from perhaps very old remnants? Such events if ever could as well have happened 3 billion years ago as now.