The chance of discovering some form of previously unknown life decreases as the availabe area in which to search decreases. The percentage of Earth explored - let's, just out of our posteriors, say, 65% - renders the chances of finding some previously unknown life form somewhat low. That 65%, by the way, takes oceans into account as well, so let's say, for land, we're talking closer to 80%.
Meanwhile, we've explored less than one one-millionth of our universe. Probably far less than that. This means the chances of finding some previously unknown life form is considerably higher - though, of course, the vastness of space and limited technology of our planet lowers that chance, as well.
The chance of there BEING such a previously unknown lifeform in the universe is exceptionally higher than there being a previously unknown lifeform on earth.
On earth, the chance of there being a previously unknown lifeform is, in general, pretty good - when you're discussing small, nearly microscopic oceanic life, for example. But get specific - say, claiming that a slightly-larger-than-man-sized primate is hiding in the northern woods of North America or the shrinking swamps of Florida - and the chance is so low that it is safe to claim such a creature is 'highly unlikely'. Never impossible - no, never close the doors to possibility.
In space, though, even if you want to claim that life is a one-in-a-trillion chance happening, it's almost a certainty. Not a complete certainty - never close the doors on impossibility, either. But we don't even know for sure what constitutes the primary requirements for life to exist on our OWN planet; and of those requirements we ARE aware of, we cannot in any way rule out the possibility that those requirements have come together dozens of times in each galaxy we observe, and all the galaxies we haven't observed.
Maybe once we've spread out, settled the mainstream mid-range stars in our own galaxy, put exploration teams out in the more off-centered stars, supergiants, etc... maybe then we might be justified in claiming that the possibility of finding some previously unknown form of life IN OUR GALAXY is highly unlikely - but even then, we won't be able to rule out other life forms across the universe.