|Mars once had a vast northern ocean.|
|The water evaporated and left pools like in Utopia Planitia.|
It makes sense for future Mars missions to look for signs of ancient life in the north martian pole, although most of the "ice" there would be carbon dioxide. Another place of interest to look is Utopia Planitia (Nowhere Plain), where the Viking 2 lander came to rest in 1976.
Utopia Planitia is reputably the largest impact basin on Mars and is likely one of the last places on the planet where water remained. It's like a giant pothole with puddle left behind by a rain storm. This "puddle" slowly shrank in time. In a desert on Earth, such a puddle would be teeming with life. As the puddle shrinks, so do the living environment, until what's left is a small area where dying critters clump together.
The Viking lander had several life-detecting experiments onboard. One of them came up positive for signs of life. Since the results could not be convincingly verified, it was concluded to be a false-positive. It's possible that it was actually the descendants of ancient life that was detected by Viking 2, but considering the toxicity of the Martian soil (it contains oxidative perchlorate), and the unhindered stream of radiation from space, it's also likely not an actual sign of life.