There is another, very simple, possibility that needs to be looked into, namely a hose clamp that needs to be "tweaked".
This exact issue was driving me crazy a few years ago with my last car. I would only smell antifreeze after the engine was fully warmed-up, and it was much more noticeable when the car was parked in my garage. There were no visible signs of leakage/seepage.
I feared the worst, but it turned out to be merely a hose clamp that needed to be tightened and repositioned.