Antifreeze leak

Why would I get about 20 drops of antifreeze when parking on a slant (my driveway), but none, when I am on a level area?

A cracked or overfilled overflow reservoir comes to mind. Or there is a leak somewhere else that is puddling and the puddle runs off when the car is on a slant.

It runs in a different direction where it can accumulate. You only get a little leak and it is enough so it will blow off or boil off while driving on the next trip. Pressure test your system to find this leak. I bet it is the water pump.