yet another problem with our explorer!!!help

We were driving down the road today and the transmission started acting up. It would not change into gear coming up a hill, finally got to the top and it started shifting normal again. Drove about 3 miles and the transmission would not work. Got out and looked and there was transmission fluid everywhere. Got pulled home and started looking we could not find a leak anywhere! Put 2 quarts of fluid back in and kept looking for leaks nothing. Took it up and down the road let it get warm and put it under strain got out and looked nothing!! anyone know if it could be the seals and if so why wont it leak now? :banghead: