Burning Odor

I received a call from a client that I did home inspection for. The inspection was a pre listing inspection. My client has owed the home for many years. And the house was in excellent shape. I only came across a few small issues. She said she has always smelled a burning odor in the house. She asked if I could come by and see for myself. I told her I would but I cannot smell (only citrus, oranges limes etc.) I asked if anyone else in the house smells it. She said her husband does not. I asked if he only smells it during the heating season (thinking maybe boiler related). She said she smells it all year round. Anyway does anyone have any suggestions (again she smells a burning all year). I am not sure how strong, and I believe it through out the whole house.

Does the house have a fireplace?