I know little about this stuff, but my impression is that there are so many different variations of life and also so many variations that can live in the most bizarre conditions, that we haven't even scratched the surface of classifying them all.
Something could survive the trip, grow, be found and still be entirely unknown to us. And it probably wouldn't be all that unlikely.
Take this study, that swabbed people's belly buttons and found thousands of unknown bacteria, bacteria only found before in foreign countries the person had never visited and "extremophile bacteria that typically thrive in ice caps and thermal vents"
Something could survive the trip, grow, be found and still be entirely unknown to us. And it probably wouldn't be all that unlikely.
Take this study, that swabbed people's belly buttons and found thousands of unknown bacteria, bacteria only found before in foreign countries the person had never visited and "extremophile bacteria that typically thrive in ice caps and thermal vents"
http://www.theatlantic.com/health/archive/2012/12/1-458-bact...