Transmission oil leak

This is a 2001 Mercury Sable V6 24500 miles:
What would cause transmission oil to leak several days AFTER being parked (I do not use the car that much). The car, after a 180 mile trip, ~70 m/h at the most, began leaking trans oil after being parked for a week without leaking (the first week, no leak at all). Then it stopped leaking, then started leaking again after a couple of days. Is it possible that there is a pressure build up by just sitting there? I presume that it leaks because some type of pressure is building in the transmission. By the way, I’m 95% sure it is coming from the spout where you add the transmission oil.
It has not leaked for some time now (~two weeks), but I’m thinking that it is because it is low.
I just took the car for a spin (~2 miles) and it appears the transmission is now slipping. It seems like at 25K miles I should not have transmission problems.
So, since the transmission was slipping, I checked the level of the trans oil. When I pulled the dip-stick, with the car heated up and the engine running; it was dry. So, I added 1 1/2 quart of oil, and the slipping stopped. I drove around for ~30 miles; then parked the car. The following day, no leak, I was so happy. Then I took the car for yet another spin later that evening ~4 miles, parked it, went into a store; when I came out, there it was the familiar puddle of transmission oil again (my 1 1/2 quart of oil was all over the parking lot). Is there something that supposed to release pressure in the transmission and it is not doing its job, so it causes it to leak when it is parked. I read somewhere that if the breather port is clogged, it could cause it to leak through the spout. But, I do not know if this car is equiped with a breather port/hole.

Please help
Bill