It's 20 degrees and 1000mbar in my living room. No brine though.

Point being, just because a set of environmental conditions which would temporarily allow brine to exist, that doesn't mean it will. At the very warmest, you'll never be far from the triple point, ice isn't going to melt in an instant (it takes an ice cube a long time to melt in a 4 deg C refrigerator) and if it's warm enough to melt it'll probably be warm enough to sublimate. It's a very small wedge in the phase diagram for liquids at that temp and pressure regime.

COULD Phoenix see liquids. It's physically possible. Will it? I don't think so. Why would Phoenix find liquid brine, when orbital assets have not? Phoenix will have a few square metres to investigate. HiRISE, CRISM, MOC, HRSC, THEMIS, CTX have covered literally millions of square kilometers with every observation with no obvious liquids visible. Why would Phoenix be any different?

Doug