The OMEGA spectrometer on Mars Express only has a resolution of about 300m per pixel so would not be able to resolve aqueous weathering due to small and transient ponds, and you acknowledged there could also be small, localized flooding that occurred in the latest geological era.
And this report says that as of the 2006 LPSC, only 5% of Mars had been mapped by OMEGA at the 300m resolution scale:

TWO YEARS OF SULFATE MAPPING IN VALLES MARINERIS AND TERRA MERIDIANI AS SEEN BY OMEGA/MARS EXPRESS.
Aline Gendrin1,2, Jean-Pierre Bibring1, Cathy Quantin3, Nicolas Mangold4, Stéphane LeMouélic5, Ernst Hauber6, Brigitte Gondet1, Yves Langevin1, François Poulet1, R. Arvidson7 and the OMEGA team.
Lunar and Planetary Science XXXVII (2006) 1872.pdf
http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1872.pdf

CRISM on MRO has a better resolution at 20m/pixel but has only started mapping Mars. I'm predicting that once CRISM has mapped large portions of Mars and specifically areas that show geomorphological indications of recent liquid water that evidence of recent aqueous weathering will also be detected.
Also, here's a preliminary report from the 2006 LPSC that suggests sulfate and talc, both derived from aqueous processes, may have been detected in Cerberus Fossae by OMEGA :

SPECTRAL ABSORPTION BAND MAPPING AT CERBERUS FOSSAE USING MARS EXPRESS OMEGA DATA.
A. J. Brown, SETI Institute, 515 N.Whisman Rd, Mountain View, CA 94043. ****@seti.org.
Lunar and Planetary Science XXXVII (2006)
http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1477.pdf

Recall, that Cerberus Fossae and Athabasca Valles show evidence of some the youngest lava flows and subsequent catastrophic flooding on Mars, perhaps within 10 million years ago:

Floods at Mars' equator are recent, scientists say.
UNIVERSITY OF ARIZONA NEWS RELEASE
Posted: February 19, 2002
http://www.spaceflightnow.com/news/n0202/20marsfloods/

Bob Clark