Burnt rubber smell, but only when heat is running - doesn't seem to be coming from the battery

At about 52000 miles, my 2010 prius II has developed a strong burnt rubber, chemical odor, but only when I run the heat. If I don't run the heat, I don't really notice the odor, although it might be there at a very low level - my sense of smell is slightly decreased with age. I read with great interest Hobbit's fascinating post about the black rubber vent tube on the battery breaking down and causing odor. However, the odor is not coming from the back seat vent, nor can we smell any odor when we took off the trunk compartment cover to expose the battery - although we did NOT open up the battery itself. Anyone with any ideas?