What I meant was that there was a seasonal change seen by Mars Odyssey in the hydrogen signal, which we interpret to be water in some form, ice, liquid, chemically bound form, etc.
One question is whether this *observed* seasonality is solely due variations in the CO2 cover or to actual changes in the amount of water in the near surface.
I wonder also if they can directly measure the amount of CO2 ice on the surface and subsurface or do they only infer a decreasing amount by an increased hydrogen signal.

Another thing I'm puzzled by is the lack of a signal in the Hellas basin in the prior measurements, especially since there is a signal for water in other regions at similar latitudes. Was this the case for all three instruments in the GRS suite or just the gamma spectrometer?
There are several lines of evidence indicating water/ice in Hellas, such as for example glacial landforms that appear to be recent and probably current, the existence of variable water vapor over the basin probably due to diurnal evolution from the surface and the existence of debris aprons that appear to be recent:

DEBRIS APRONS IN THE EASTERN HELLAS REGION OF MARS: IMPLICATIONS FOR STYLES AND RATES OF HIGHLAND DEGRADATION.
Lunar and Planetary Science XXXIII (2002).
"Introduction. Geologic mapping and geomorphic analyses have revealed a concentration of lobate debris aprons in the eastern Hellas region of Mars [1-8]. These debris aprons are part of a suite of features found at Martian mid-latitudes that are considered indicators of ground ice [6,9-11]."
http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1642.pdf

Perhaps there is a seasonality to the water in Hellas as well, but in this case it would be low in Summer. But why would this be true in Hellas and not other regions of similar latitude and why low in Summer?
A speculation: perhaps the low elevations and relatively high atmospheric pressures in Hellas allows the water to be in liquid form. This allows the liquid water to migrate to be closer to the surface. Then this water evaporates when the temperature rises.
For other regions at similar latitudes the water stays in frozen form and does not migrate close to the surface.
If true then there should be a higher signal for water in Hellas as the temperature drops.

Bob Clark