It took the modern astronomy to live to the year of 2014 to discover 2014 UN2711 (sized roughly 100-200 km and now being re-qualified from a comet to mini object), which has "extremely" elliptical orbit with the ~600,000 years period and the next nearest to Earth position to be in 2031. Note that in this case the lead time between the object discovery and it's reaching the closest to Earth position is 17 years... So perhaps eventually other objects with such elliptical orbits and periods exceeding the historical age of the observational astronomy which itself is not to exceed 10,000 years, counting from human naked eye intelligent ability to observe celestial bodies could be discovered in the future? How large in size those objects could be, how close to Earth those objects could pass, what are the probabilities of such objects to hit the Earth and what could be the range of the lead times for such objects between their discoveries and them reaching the closest to Earth position? Could it be assumed that there is a probability of existence of undiscovered yet Oort objects with elliptical orbits and with size/weight parameters being equal or greater than such of Mercury (4,879 km across and 330 trillion tons correspondingly)?
The team of scientists lead by Marshall Eubanks of Interstellar Exploration Initiative organization published the study in Naked Science magazine in March 2021, which estimates that about 7 interstellar objects pass through the inner solar system every year.
1From Earthsky.org's Mega Comet Inbound From Oort Cloud
The newly discovered mega comet, 2014 UN271, is currently diving from the outer solar system. At its closest to our sun, it’ll come close to the orbit of the outer planet Saturn in 2031