Thermal Vents on South Sister

I happened to be up on the South Sister the other day during what was probably the first snow of the year. From the summit I decided to do some exploring and headed over to Hodge Crest. Half way over I put my hand down on the rocks and felt warm air venting through the lava rock. I have never heard of any activity on that mountain whatsoever. I checked twice and then on the way back to the caldera rim, it was definitely blowing hot air out of somewhere.

Does anybody have any info on this?