Curiosity is loaded with sensors, these can be used to draw conclusions they weren't designed for. Here's an admittedly absurd example: if Curiosity's cameras recorded a Martian puppy running around it, scientists would almost certainly consider that conclusive evidence for life.
It would be evidence but not conclusive evidence. In this case, we might think it was some sort of horrible prank played by a disgruntled scientist who hacked the instruments. But it might be enough for a second mission designed to conclusively verify the existence of life.
What if the Martian puppy started licking Curiosity and started to reprogram the cameras and antennas to communicate with us like an interplanetary telephone system?
That would be a pretty conclusive proof of live even though Curiosity wasn't designed to work in that way.
