Nick Hoffman proposed an area in the Hellas basin that may contain hydrothermal systems:

Or maybe it did...
Posted by Nick Hoffman (61.9.128.174) on 5/1/2003 4:32:52 PM
In Reply to: To those who were hoping to find 'active' hot-springs on Mars... posted by Jacob Samorodin on 5/1/2003 8:38:00
http://habitablezone.com/space/messages/33823.html

In his book _Mars: the Living Planet_ , Barry DiGregorio discussed some instances where scientists had earlier proposed active volcanic vents or geysers on Mars.
Leonard Martin observed a rapidly expanding cloud in two separate instances in Viking orbiter imagery, discussed in Barry's book on pp. 211-214.
The first was in an area near Solis Lacus, or Solis Planum, on Mars. This is interesting because Solis has been observed to present anomalously high water vapor levels. It has also been found to be the point of origin of some of the global dust storms, which some scientists attribute to rapid, extensive volatile release, likely H20, in Solis.

Martin's observations are described here:

Aug. 9, 2000
Is Mars water old news?
By Billy Cox
A FLORIDA TODAY column
http://www.floridatoday.com/news/columnists/cox/080900cox.htm

Martin observed this in Viking orbiter images 775a10 and 775a11 which are reproduced here:

http://www.lauralee.com/JPL.htm

I've been unsuccessfully trying to find MGS images of the specific spot where the proposed venting occurred, if anyone can find any. The Viking images are centered at 16 S latitude, and 80 W longitude, but the spot is to the lower right in these images.
The second area was near the Arsia Mons volcano on Mars. The initial event was observed in Viking image 439a25 and was seen to later develop into a growing cloud in image 439a44. Because of some unique topography I was able to identify where this proposed venting occurred in an MGS image:

Sample flows near summit of fan-shaped rise off S Arsia Mons
http://ida.wr.usgs.gov/html/m22020/m2202078.html

In the Viking context image on this page, the cloud developed from the sinuous fissure that is partly covered by the box indicating the coverage of the MGS image. That the cloud developed out of the fissure would be consistent with its being due to volcanic venting.
There is a curious feature of the MGS image on this page. There is a blurriness in the image. This might be due to low level fogs over the area, which would be consistent with venting, but the blurriness rather gives the impression as being due to motion. Perhaps it could be due to spacecraft motion, but the blurriness does not extend over the entire scene. Quite curiously some features are protrusions that seem to have the blurriness due to motion (you might need to rotate this image to properly perceive positive and negative relief.)
On the other hand perhaps these protrusions could be Nick's ice towers that are emitting vapor, which is causing the blurriness. They do have a white color to them.
Another scientist who proposed there might still be active volcanic vents on Mars is Baerbel Lucchitta, a noted authority on Valles Marineris on Mars. In Viking imagery she observed dark-patches on the canyon walls, which she interpreted as recent volcanic vents:

Volcanic Features in Valles Marineris Reexamined with MGS Data.
Lucchitta, B. K.
34th Annual Lunar and Planetary Science Conference, March 17-21, 2003, League City, Texas, abstract no.1752
http://adsabs.harvard.edu/cgi-bin/bib_query?2003LPI....34.1752L

Mars: Volcanism in the Valles Marineris overlooked.
Lucchitta, B. K.
Lunar and Planetary Inst., NASA MEVTV Program Working Group Meeting: Volcanism on Mars p 13-14 (SEE N88-30487 24-88)
http://adsabs.harvard.edu/cgi-bin/bib_query?1988LPICo.660...13L

More on RECENT(?) Dark Volcanic Patches in the Valles Marineris, Mars
Lucchitta, B. K.
LUNAR AND PLANETARY SCIENCE XVII, PP. 496-497.
http://adsabs.harvard.edu/cgi-bin/bib_query?1986LPI....17..496L

The sites of apparent venting observed by Martin and the dark-patches on Marineris walls proposed to be volcanic vents by Luccitta would be good places to check for anomalous temperatures using THEMIS.

Bob Clark