Nick described how to read the THEMIS brightness images here:

How to do it with NASAVIEW.
Posted by Nick Hoffman on 8/7/2005 6:02:09 AM
In Reply to: re: Martian high north latitude hot spots? posted by Robert Clark on 8/6/2005 5:06:41 AM
http://habitablezone.com/space/messages/390308.html

He discussed this image:

THEMIS Image Data Page for I02026005.
CENTER_LATITUDE = 39.1376
CENTER_LONGITUDE = 251.959
MINIMUM_BRIGHTNESS_TEMPERATURE = 302.213
MAXIMUM_BRIGHTNESS_TEMPERATURE = 322.192
http://themis-data.asu.edu/img/I02026005

He observed the hottest areas were in the Sun-facing walls in the image, which argues against the maximum temperature being due to a hot spot.
But a key fact about this image is that even the minimum temperature is quite hot for this latitude. Do a search for instance on http://themis-data.asu.edu . Click the Advanced button, then search on the IR images in the latitude range 37 to 41. You'll get a couple of thousand images. The highest maximal temperatures you'll see will be overwhelmingly in the 250's. There are only perhaps 2 or 3 in the 300's. So this particular image even has it's minimal temperature 40 to 50 degrees hotter than most other maximum temperatures at that latitude.
In reading some articles about the brightness temperatures it turns out they are affected by albedo as well as surface temperature. So you might guess this is an unusually bright area. But the corresponding visual image does not give that impression:

THEMIS Image Data Page for V02026006.
http://themis-data.asu.edu/img/V02026006

It would be interesting to find out if current or past Mars Global Surveyor TES imaging also shows this to be an unusually warm area.

Bob Clark