Wife's car does the same thing. She works 3 miles away. Gets home and it's like the car burps a small puddle on the garage floor. Will loose like 1/2" cup of coolant. Anyone know what's up with this leak. I might try replacing the thermostat and also drill a small weep hole to help pressure equalize.
I wonder if it could be tightened (I'm sure you already tried) or some kind of seal be placed around it. The other options might prove pretty expensive unless you're still under warranty.