On Earth water condenses and forms clouds. There is very little of it at the top of the atmosphere. Instead we see atomic oxygen and ozone.

>The one issue I have with this hypothesis of deep oceans that have evaporated away is the apparent lack of surface evaporite deposits. Where's the salt flats?

https://en.wikipedia.org/wiki/Vastitas_Borealis

>"Results published in the journal Science after the Phoenix mission ended reported that chloride, bicarbonate, magnesium, sodium, potassium, calcium, and possibly sulfate were detected in the samples"

Not too far off from "salt flat".

Mars has extensive dunes and dust storms. The frost reworks water soluble material. It should not look exactly the same as Earth's salt flats.