> Also very interesting are the fast neutron maps showing some areas
> with moderately high hydrogen amounts. The fast neutron measurements
> are most allied with the near surface hydrogen amounts, within a few
> centimeters.
>
> See the maps of fast neutrons in these reports:
>
> ATMOSPHERIC CORRECTIONS FOR NEUTRONS REVEAL VARIATIONS IN SURFACE
> COMPOSITION IN THE THARSIS REGION.
> T. H. Prettyman, W. C. Feldman, W. V. Boynton, G. W.
> McKinney, D. J. Lawrence, and M. T. Mellon.
> Lunar and Planetary Science XXXIV (2003)
http://www.lpi.usra.edu/meetings/lpsc2003/pdf/1950.pdf
>
> Microsymposium 36, MS070, 2002
> FIRST RESULTS OF MARS MAPPING OF NEUTRON FLUX BY HIGH ENERGY NEUTRON
> DETECTOR, MARS ODISSEY.
> I. G. Mitrofanov, M.L. Litvak, A.S. Kozyrev, A.B. Sanin, W. Boynton,
> C. Shinohara, D. Hamara, R. S. Saunders.
>
http://www.planetary.brown.edu/planetary/documents/Micro_36/Abstracts/070_Mitrofanov_%20etal.pdf
>
> These images though don't give the seasonal differences for this close
> surface water so you can't tell if there is a seasonal and/or diurnal
> variation, which would indicate free water quite close to the surface
> near the equator.
>
> In Fig.1 of this first report, there is a strip of dark blue
> indicating significant amounts of hydrogen at around -10 to -20 S.
> lat. and -165 E long. There are also scattered, small light blue areas
> indicating moderate amounts of hydrogen.
> These would also be good places to search for seasonal changes in the
> MOC images.
>...

Here's another map of fast neutrons, which are most allied with
hydrogen in the topmost layer of soil:

VERTICAL DISTRIBUTION OF SHALLOW WATER IN THE DISTINGUISHABLE REGIONS
AT LOW
AND HIGH LATITUDES OF MARS: NEUTRON DATA DECONVOLUTION OF HEND.
I.G. Mitrofanov, M.L. Litvak, A.S. Kozyrev, A.B. Sanin1, V. Tretyakov,
W.V. Boynton, D.K. Hamara, C. Shinohara2, R. S. Saunders, D. Drake, R.
Kuzmin
http://www.lpi.usra.edu/meetings/sixthmars2003/pdf/3080.pdf

See Fig. 2 here. This does not give the seasonal difference in the
fast neutron amounts so you can't tell if this is free water or
chemically bound water.
However, there are a few scattered areas of light blue to dark blue
in the near equatorial region indicating moderate to significant
amounts of water in some form in those locations.
One particular area of dark blue is located slightly west of 120
degrees E long. and slightly south of 0 degrees lat. Anyone from the
HEND data able to better constain the coordinates of this site?

Here's a map of MOC images covering this general area:

Area between -15°and 0° Latitude and 240°and 255° Longitude
http://ida.wr.usgs.gov/html/areasg/c_07s247.htm

Here's one image from this area:

Transition between deep and shallow portions of valley.
http://ida.wr.usgs.gov/html/e13022/e1302254.html

Note that there are whitish deposits in the lower elevation regions
in the image, in the craters and channels.
In one of the craters there is the definite appearance that this
white material flowed down the sides to the bottom of the crater: