Deeming it contamination is not automatic. We don't even know if the bit is actually contaminated (D3 Gold).
But it would cast doubt on the findings.
However, as invivo points out, there could be ways to rule out the possibility easily, and the prospects of contamination is arguably pretty slim, so finding life despite the concerns that Curiosity (or indeed other landers, due to the small but ever-present risk they were contaminated) still presents a credible case for believing it originated on Mars, especially if it's found by another spacecraft miles away from Curiosity.
Another possibility to add to those suggested by invivo is that life could be found buried deep in the soil, which provides protection from the harsh radiation environment at the surface. In such a case, if the life is found to be incapable of surviving at the surface, that also tends to rule out contamination.
That doesn't really matter in the analogy above. Sound and light travel at finite speeds (D3 Gold), and in roughly straight lines unless they encounter interference - and I think we can ignore everything but ray optics and its analog on acoustics for this thought experiment.
If I drag a stick across a pond at the same speed that the ripples propogate at an observer at a point off of the path of the stick will eventually feel the ripples - the stick will have met and left the closest point of approach when the ripples are felt but they will still feel the ripples. The same would be true for hearing a jet passing by at mach 1 (whatever mach 1 happens to be for the medium you are in) and for the hypothetical object traveling at c (or below c in something other than a vacuum).
Actually it occurs to me that anyone who has ever gone sailing has probably felt the pond analogy (D3 Gold): you are happily sailing along sipping your vodka tonic and a power boat comes racing by, the boat passes by and you have time to hold your drink away from your self and hold on (one hand for your drink, one hand for the ship) in preparation for the oncoming waves to knock your boat about. The speedboat was traveling faster than the waves but the sailboat sure as heck felt it!