well, as you probably know you can't even count the amount of stars and most aren't even discovered yet seeing as their light didn't reach earth yet
from so many plants the odds a life form of any kind does not exist are so small that anyone at all who thinks there isn't any life form anywhere is nothing but a complete moron.
though no alien ever got to earth, if you had the ability to travel faster than light speed by far thus being able to actually reach planet earth, would you even care about some lousy race that kill itself and can only reach it's own moon? i doubt..
if anything of that sort reach earth by some weird error the odds for it to care enough to stay are too low
if that life form does stay however you can be more than just sure that you'll know about it cause such life form will probably not like it if say the goverment of the human apes planet kidnapped one of them
bottom line:
they exist, but they won't come here, i even doubt their ability to do so cause no one said they are that much brighter than us, they might as well be as dumb or maybe even dumber