2004 Mazda 6 – Need help narrowing down the issues

Hey everyone. My first post; please be gentle!

My girlfriend recently moved in with me and she has a 2004 Mazda 6 S(3.0 V6, Auto). She was having issues with the car so I decided I’d take a look. Her list of complaints were as follows:

1.) Smoke inside the dash.
2.) Fog inside the windshield.
3.) A/C and Heater issues.
4.) Engine hesitation during light throttle.
5.) Coolant smell inside the car.
6.) Coolant leak outside the car.

Immediately what sprang to mind was the heater core. Either an failure or a blockage. I went to the car and drove it around the block a couple times to see the issues for myself. I didn’t notice a thing except for the coolant smell. Heat worked. A/C worked. I took the car to my mechanic, and he said he refilled the coolant but beyond that he didn’t experience any issues…

Next day I did it again, and I did see the issues she mentioned all at once. Very strong coolant smell, smoke in the cabin, heat not working, etc. There are no check engine lights. I grabbed my scan tool and decided to check anyway. The recent codes looked to be:

U0101, U0121, U0155, P0421, P0304.

I asked her about recent service, she couldn’t give specifics but pointed me to the glovebox full of receipts from various places. Midas, Pep Boys, Meineke, the dealership. The most recent I could find from Midas is 11/23/2015 but nothing to do with the issues above. (Control arms, brakes, alignment and transmission fluid. ) She did say she had the radiator replaced a couple months ago because the car was overheating… ( I warned her about maintaining her car properly..)

Today, before posting I wanted to check the car again and behold, no issues what so ever again. Just the coolant smell inside the car. No check engine lights, no heat issue, no a/c issue. I’ve never seen such an issue as intermittent.

I’m thinking the heater core is the place to start with, any ideas regarding the issues? Any help you can give is appreciated.