Lake Vostok
That should have been local and deep-seated water.
You can actually map out Lake Vostok and a few other sub-icecap lakes by the smooth surface topography of the flowing ice (no basal drag). This can, in principle, be mapped from orbit using radar or laser altimetry.
People have looked for equivalents on Mars but no joy yet for reasons that I think are amply explained in my response to Jim O.
You're welcome to your own opinions, of course, but I've actually done work on this topic and been to the conferences.
And in closing, I'll add that I would really like to be proven wrong, since it will make Mars a more interesting place. (Opportunity has already done some of that, but not quite in the way most people imagine).