What about "Biologically Reversible Exploration"???

"The international Committee on Space Research (COSPAR) has established a "planetary protection" policy that involves not contaminating other worlds in a way that would jeopardize the conduct of future scientific investigations. As a signatory to the 1967 Outer Space Treaty, the United States is required by article IX to avoid "harmful contamination" of the other worlds of the Solar System. However, further revisions to the policy are needed...

The spacecraft that have landed on Mars have all been surface missions. Contaminants will remain local and static and can be removed without requiring an effort vastly larger than the missions that carried the contamination. Even at the crash sites, debris from Earth extends no more than a few meters into the surface. Reversing the contamination involves recovering the spacecraft parts and exposing any contaminated dirt to the sterilizing ultraviolet (UV) sunlight. However, if, for example, robotic or human explorers drill to investigate a subsurface aquifer, biologically reversible exploration would require rigorous sterilization of any components that go down the drill hole. Similarly, if human explorers establish bases inside caves (12), the naturally sterilizing effect of the surface UV would be lost, and contamination would be persistent.

We should not do anything now that would close off options for the future. I propose that COSPAR, in its upcoming discussions, set a policy that all Mars exploration be biologically reversible and that this policy extend to human exploration as well."

http://www.sciencemag.org/cgi/content/summary/323/5915/718

The main point of the Schon et al. article is on the timing of gully activity, not simply the presence of liquid water. We know from numerous morphological and mineralogical lines of evidence that Mars once had a more temperate climate, but that was > 3 billion years ago....

This study points to flowing water (however transient) during the last period of higher obliquity, about a million years ago. -- the blink of an eye in geologic time.

http://geology.gsapubs.org/cgi/content/full/37/3/207

What about extremely large debris-covered glaciers? Would that do the trick for you?

The SHARAD instrument (really cool, 15Mhz radar) has shown conclusive evidence of ice cores in features called lobate debris aprons. These debris-covered features have probably been stable for ~100 Ma.

http://www.sciencemag.org/cgi/content/abstract/322/5905/1235

abstract: Lobate features abutting massifs and escarpments in the middle latitudes of Mars have been recognized in images for decades, but their true nature has been controversial, with hypotheses of origin such as ice-lubricated debris flows or glaciers covered by a layer of surface debris. These models imply an ice content ranging from minor and interstitial to massive and relatively pure. Soundings of these deposits in the eastern Hellas region by the Shallow Radar on the Mars Reconnaissance Orbiter reveal radar properties entirely consistent with massive water ice, supporting the debris-covered glacier hypothesis. The results imply that these glaciers formed in a previous climate conducive to glaciation at middle latitudes. Such features may collectively represent the most extensive nonpolar ice yet recognized on Mars.

Liquid water can be metastable at close to current orbital parameters (Mars' obliquity - the tilt of its orbital axis, varies significantly).

This was investigated by Hecht (2002): doi:10.1006/icar.2001.6794.

Abstract:
A simple model of local heat transport on Mars demonstrates that transient melting of ice may occur in depressions and gullies nearly anywhere on the planet where thin ice is illuminated by normal-incidence insolation. An experiment has been performed to confirm the model of evaporation rate at low pressure. Reduction of radiative cooling due to gully geometry is shown to be important. Since appropriate meteorological, topographic, and optical conditions may occur on slopes nearly anywhere on the planet, hydrological features such as gullies would likely form only where such ice might accumulate, notably in sheltered locations at high latitudes. It is suggested that cold-trapping of winter condensation could concentrate a sufficient amount of ice to allow seasonal melting in gullies.