A leak in the system that's burning off could cause that. If you have a heater core failure, you'll usually have copious amounts of coolant steam in the cabin. It's caustic enough to be unmistakable.
Even a small leak under the hood can migrate fumes into the cab through the A/C/Heater vents. The last time my water pump failed I noticed some odor first. The water pump failed from a shaft seal leak.