Coolant smell.. No leak??

[MookaMobile]

This forum is very helpful. I have another question, maybe someone else has had this happen.

Occassionally while driving, I get that sweet, tangy odor, usually associated with a coolant leak. It seems to happen only when it is cold outside and usually when I have the heat on (fan on high-still need to replace resistor).
I took it in for a leak check, the mechanic thought maybe I had a bad heater core or leak, but upon inspection and pressure test, he found no leak and the heater seems ok. He did say coolant was about half and cup low, which could just have happened over time, as I have not added any in the 2 years I have owned the car, and it was topped off when I got it.
The car hasn't done this before, and we can't find anything obviously wrong with it. Is this intermitten odor normal?

We're planning a 1400 mile round trip journey in the next couple weeks, and I am worried about my baby as usual. It first started having the odor when I had to drive it 700 miles from SC to OH in November, and has been off and on since then.

Thanks!!