If anyone is looking for a natural explanation, this appears to be a cold weather fumarole Cool surrounding air is drawn into the lower levels of the Grotto where it picks up a little moisture as it is warmed. The warm air rises, cools, the moisture condenses and under favorable conditions becomes visible as a slowly rising fog. Apparently this particular cloud was not dense enough to be noticed by the writer, but the physics of the camera was able to detect it.
Somewhat similar phenomena resulting from fortuitous configurations of small animal dens, road culverts and other drainage systems can sometimes be seen in the cold of winter. Also related is the early morning mist arising from warm bodies of water after a strong cold front has gone through, mostly seen in the fall. Wind will quickly dissipate any of these things.
The “physics of the camera” which brought out the unseen plume was likely the camera’s flash.
All in all, an impressive scene and unexpected occurence.