Coolant smell from vents, particularly on drivers side?

Since November, I’ve had a coolant smell in the cabin when the heat is on. In late November, I took it to my trusted mechanic due to the smell of coolant / exhaust when idling. He pressure tested my car but was unable to find leaks. He suggested I replace the cabin air filter. This fixed the exhaust like smell, but I’m still getting a coolant smell, but only when the heat is on & even then it varies. Sometimes it’ll smell like coolant, or a burning smell, and other times, I won’t smell it at all. I also at times smell coolant on the driver's side (with the hood closed). I don’t have any leaks on the ground and tried UV dye. I noticed a bunch of neon green spillage on the water outlet, but after further inspection, this seems to be from when the dealership spilled oil when doing an oil change. (I confirmed this as there was a small puddle of neon green on a small block and rubbing my fingers on it confirmed it to be oil). I’m a bit lost. I don’t think it’s heater core as my heat blows strong and hot. My passenger floorboards aren’t moist, and I’m not getting window fog.