is located on the vast Tharsis volcanic uplift, a vent of some sort would make sense. Some have argued that the Tharsis volcanoes are routinely venting steam even now (partially accounting for the clouds so often found at their summits), and additional vents on the Tharsis plains shouldn't be too surprising.

I fail to see the rille you point out (could be my failing eyesight!) though there is a dark apparent shadow related to the cloud. Of course, low-lying fogs are common on Mars. If there is a depression at that location, then fog could well be responsible. I'll take a look at some of the other images of the region, see if any formation seems to correspond with this potentially very exciting "whats it."

Cheers, B R Felix