On prolonged sitting, enough fluid can drain out of the torque converter to spill out the vent -- you can lose up to a quart this way. Looks way overfull or as if you have a bad leak, but it's just overflow.
This drives my mechanic friend nuts -- he has a customer with a 68 280SL, beautiful car, but he refuses to drive it except to bring it in to "get the transmission leak fixed" every year or so when it starts to drip again. Totally preventable by just starting the bloody thing once a month or so, but hey you can't tell some people anything at all!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!