The probability of aliens resembling humans from another planet being undetected on Earth would depend on several factors, including the technology and capabilities of the aliens, their intentions, and the methods used to detect them.
If the aliens had advanced technology and intentionally tried to remain undetected, they could avoid detection using stealth or camouflage techniques beyond our current detection ability. Additionally, if the aliens were only observing us from a distance or only intermittently visiting Earth, their presence may go unnoticed by humans.
On the other hand, if the aliens were actively trying to interact with humans or establish a presence on Earth, it would be more difficult for them to remain undetected. In addition, there are numerous methods that scientists and researchers use to detect signs of extraterrestrial life, such as searching for radio signals or analyzing atmospheric compositions, and it is possible these methods could detect the presence of alien life even if they resembled humans.
Overall, it is difficult to estimate the probability of aliens resembling humans from another planet being undetected on Earth without knowing more about the specifics of their situation and capabilities. However, it is worth noting that the search for extraterrestrial life is an active area of scientific inquiry, and researchers are constantly developing new methods and technologies to improve our chances of detecting signs of alien life.