Such a false negative could lead to imprudent risks on premature human missions. A wise policy would be to plan a series of orbiters and robotic landers before bringing a sample back from a carefully selected site. Such a policy will create public confidence. It is plausible that life started on Mars during an epoch when there was free water on the surface of Mars. If life ever did start on Mars, organisms would have developed to occupy surprisingly hostile environments and utilize unusual and sparse resources.
Life can use rather exotic ways of acquiring and storing water resources.
In addition, Thomas Gold has conjectured that life may begin deep below the surface. The search for extant life on Mars should begin with a thorough global examination of possible ecological niches. Of particular concern is the distribution of water. As much as 500 meters of rock and soil may isolate Martian ground ice near the equator from the atmosphere. Accurate instruments are not yet available for measuring the depth and horizontal distribution of water.