2000 Ford Taurus Transmission issue?

I have just purchased a 2000 ford taurus with a little under 136,000 miles. When I test drove it I noticed that it seems like the front left wheel bearing needs to be replaced other than that seen no issues. However on the drive home of about 50 miles at around the 40 mile mark the person following me signaled for me to stop. I quickly seen why as soon as I had pulled over smoke came rolling around the car. I checked under the hood seen it was coming from under the car. Checked under the car and at least looked like transmission fluid was pouring out. Being so low to the ground was difficult to see where from. I ended up towing the vehicle the rest of the way and getting into my other cat as I was running late for work. The next day I start up the car to start to find the issue. No leak. Checked the transmission fluid level it was low so filled I up and continued the search. Still no leak. I let the car run for over two hours while up on ramps. No leak. I've driven the car short distances of 10 miles or so let it run and idle for hours on end without ever seeing another leak. Is this some sort of fluke? Is there something that might happen after driving for an hour or so that will cause a leak that idling alone won't? I'm too paranoid about it to drive it too far and really need to figure this out as my other car has one foot in the grave. Any ideas appreciated thanks in advance.