I finally found a dealer service department who would take the time to answer and who knew the answer. They said that it is a vacuum reserve, used to move things like the shutters in the passenger compartment air distribution system. This would explain why the heat seemed to keep coming from the floor, even when I requested the panel. This also explains why one of the hoses appears to go through the firewall. It probably does. Good that I found this before summer, when I need the AC!