Over the last month, When I get in my car, I notice a Anti Freeze odor. It only smells for about 15 seconds, then it is gone. It has happened 3 times. When I notice the odor, I have not run the A/c, or heater from the last time I drove the car. I felt the carpet on the passanger side to see if it is damp, and it is dry, and I did not see any fluid leaks under the hood, or on the ground. Any thoughts? Is my heater core starting to go and just have a small hole in it? My anti freeze level has not changed either.

Thanks