Okay I also think Mawrth Vallis seems a good place - especially since their rover's range of 3 km or so is much smaller than it's landing ellipse.
I think though that it only goes the one way. They've got a good chance of finding life in the clays of Mawrth Vallis, perhaps as good as any early life deposits. But if they don't find it, then it won't show that life did not emerge on Mars.
That's one of the things I go into a bit later in the article and I think I might do it again as a separate article to bring attention to it. Because, we don't know how long it took for life to get to the point where it can survive almost anywhere. For instance when did it first develop photosynthesis? Without photosynthesis then it would be very limited in its range.
In Noachian period Mars it's had only a few hundred million years to get started. And - there are about as many evolutionary steps between the first primitive life and the archaea as there are between the archaea and us, at least as far as nucleotide complexity is concerned.
So, either life originated somewhere else around another star - if so then is almost certainly on Mars also and using DNA also as for Earth - or else - it went through those evolutionary steps very rapidly on both Earth and Mars. In the first case yes expect to find it in Mawrth Vallis. But if we don't find it there, it would show pretty conclusively that the life can't have been delivered to earth as archaea from another planet around another star.
That then would mean it went through those early stages of evolution with amazing rapidity. If so - how far did it get in early Mars before the end of the Noachian?
That then depends on how you think life began. If it began in hydrothermal vents for instance and was still confined to them by the end of the Noachian, then you wouldn't expect to find them in Mawrth Vallis. We might need to go to Nili Fossae (say) or Opportunity's Endeavour crater rim possibly or some such.
But is no guarantee of success to target the hydrothermal vents either because that's only one possible habitat. Another one is eutectic freezing, if so maybe should target the Noachian period eskers in the Dorsa Argentea Formation around the S. pole.
Or might be that life started deep underground. If so the best place to target is Leighton Crater or McLaughlin crater.
Or, might be it started in some more surprising place, for instance on pumice stones (minority view of possible origin of life). So we might need to look for evidence of Noachain age pumice.
Mawrth Vallis seems one of the best places because of its layered clay deposits, and if life is widespread in early Mars then we hope to find it.
But I think need to be careful not go give the message that if we don't find it then it's not there.
Also if there is no ancient life on Mars (I think that's unlikely) there must at least be pre-biotic chemistry so you get to see what happens on a planet with a sea rich in organics for hundreds of millions of years. Must have at the least pre-biotic chemistry and protobionts as you get those easily in laboratory experiments.
So, let's hope that ExoMars does find unambiguous evidence of early life - that would be really exciting for everyone. But if not, we shouldn't give up on the search because it might be a long one and we might need to search many different places, I think there is a reasonable chance that life in the Noachian period is not very developed yet and only exists in a few specialist habitats. And not even all of them. E.g. if life evolved in one of the hydrothermal vents on Mars but is not yet robust like modern Earth life, then it might take many millions of years before it is able to transfer to another hydrothermal vent in the same ocean and indeed might evolve differently in different hydrothermal vent areas, and similarly also for some of the other suggestions.