The lack of a signal from Mars Odyssey at the high northern latitudes of Mars for water was attributed to a CO2 cover during the winter months.
This appears to be verified by an increasing signal for water at the high northern latitudes as the temperature warms:

DPS 34th Meeting, October 2002
Session 11. Mars Odyssey Symposium
http://www.aas.org/publications/baas/v34n3/dps2002/S110.htm

11.02 The stable and variable features of subsurface water on Mars: HEND data from Odyssey
I.G. Mitrofanov, D.S. Anfimov, A.S. Kozyrev, M.L. Litvak, A.B. Sanin, V.I. Tret'yakov (Institute for Space Research, Moscow), W. Boynton, C. Shinohara, D. Hamara (U. Arizona), R.S. Saunders (JPL), HEND Team, GRS Team, Odyssey Team

11.03 Recession of the North-Polar CO2 Cap During Early Spring: Results from the Mars Odyssey Gamma-Ray Spectrometer
W.C. Feldman, R.L. Tokar, T.H. Prettyman (Los Alamos National Laboratory), W.V. Boynton (University of Arizona, Lunar Planetary Laboratory)

11.04 Subsurface Ice Content in the North Polar Region of Mars
W. V. Boynton, D. K. Hamara (University of Arizona, Lunar Planetary Laboratory), W. A. Feldman, T. H. Prettyman (Los Alamos National Laboratory), Mars Odyssey Gamma-Ray Spectrometer Team

Bob Clark