I don't get the idea of why they keep sending the rovers to Mars, there is already a bunch of them that work properly and look for the signs of life there and if there were the sings of life, we would find them out already.
Consider the opposite situation. If we lived on Mars and wanted to find out about Earth, it would take much more than a few probes to determine its global and local aspects. We know this because we live here, and know that Earth is rather complex after all. Mars should be expected to have that same complexity.
Earth, like Mars, has very different geological features all over the surface. It is only by investigating enough of them can we gain a more complete understanding of the planet. A reasonable guess is that we have considerably less than 50% of the most important facts so far.
It is also important to appreciate that, over time, these Mars probes are getting fitted with increasingly more sophisticated instruments as the technology evolves after each mission. With more precise and variable instruments, a more precise determination of its nature can be established.
As to definitive signs of life, they may be buried well below the surface. It could take a great effort to find absolute signs of life on Mars in the past, or in the present, if at all.
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