In this novel they find life inside on of the large volcanoes, I think it was Olympus Mons. Benford's (a physicist at UCLA I think) reasoning was that residual heat may still be there deep down which would keep some bacteria alive.

The main problem with that though is you can never get a probe or rover in to look for life there, so that will have to wait for colonisation to check. In the story they go deep inside caves, which were probably lava tubes, to find life.

Since the book was published some of the Mons have been found to have a high ice content in the soil so this is more plausible.

This is a very handy map of Mars, but a large file

I do wonder though if the bottom of Valles Marineris might be a good place to look, if it was possible to get a rover down there. The air pressure would be higher (I think nearly double), there is hematite there as well which if Opportunity finds it is made from water then it is likely to be made the same way in VM.

Geologically many believe that Valles Marineris contained lakes and there are large channels going north into Xanthe Terra and Margaritifer Sinus. The implication is that water flowed through Valles Marineris into these channels, but this is likely to be extremely ancient. Somewhere along these channels would also be interesting. Chryse Planitia where these channels point to is another good place. If you look at the channels in the map it looks like a logical place to find old water signs.

Another interesting place is Lucus Planum with high ice content in the soil, and what is described as an estuarine delta shape there. Also Amazonis Planitia is described by some papers as being smooth like an ocean floor, so it could also be good for a Rover to land on. If it was an ocean floor then it would be interesting geologically. It's still unknown how areas like these came be so flat if not formed from water.

There is also strong evidence that there were glaciers on Olympus Mons and the three Tharsis Montes, Aschraeus, Pavonis, and Arsia Mons. This is also probably impossible to get to with a lander but would be a good place for colonists to look at. In fact if there are lava tubes in this area it might be a good place to establish an initial base, inside the tubes themselves.

It may be possible to seal and pressurise them (they should be similar to earth lava tubes), and there is known to be ice in the soil there. If they are anything like Earth lava tubes they should be a natural place to build in. Kim Stanley Robinson had people colonising lava tubes in his Mars trilogy.

Lava tubes are natural conduits through which lava travels beneath the surface of a lava flow. Tubes form by the crusting over of lava channels and pahoehoe flows.