They may not even find traces of water. This was discovered from orbit, not from actual samples.
They wasted no time using this discovery to call for a manned Mars landing. I'm not saying there is no water on Mars and I'm all in favor of exploration, but this sounds more like a play for more funding then an actual discovery to me.
Indications are that 3 different instruments have detected the signs. That's a pretty good start at making an estimate in my opinion.