A bit wacky.
Ok, seems the most likely place the water went was underground.
(How could water vapor have escaped Mar's gravity?)
How did it get there?
The impact that formed the Hellas basin set up a resonance (rang Mar's bell so to speak), which caused a type of reverse liquefaction. The "surface tension" of the planet was reduced to a point where Mars became a sponge.
Just a guess
Anyone else?