Sometimes, strange odors are emitted when, say for example, you're down the basement and you're staining some wood. The odor travels upstairs and is 'burned' by the heater, thus, giving off a different smell. That's combustion.
You or your wife have have been working with 'something' that got inhaled by your ventless heater. Things like 409 Cleaner, stain, even dinner will get inhaled by these systems and 'altered' by the heating of them. That's why it's not advised to burn candles or anything when you're staining or painting or even using noxious cleaners because of the by-products from burning.
Other than this, I can't imagine what the smell would be. You can contact the mfgr, the propane retailer, or even the local fire company and see if anyone has any further insight. IF you come up with an answer, I'd certainly be interested in knowing what it was. My best to ya and hope this helps.