transmission leak

Bought a 1992 roadmaster sedan, one owner. It has 156000 miles but the transmission was replaced at 75000 miles with one from a dealership. After purchase i left it at a parking garage and started it once, next time I went by the car I noticed a very large amt [the dipstick shows empty] of what turned out to be transmission fluid under the car. The previous owner [a coworker] had no problems [recent]with the tranny, and it has not been driven at all since I purchased it [ been about a month]. So I am wondering if anyone has ever had this problem and what sorts of repairs I might be facing. Thanks for any information.

Since nobody more knowledgeable has answered I give it a try.
My first (and only...) idea is that if You have a transm.oil-radiator one cause could be lines to/from the radiator has sprung a leak. My -95 sprayed transoil at the front and it was a crack in one of the lines to the external transoilradiator (i have towpack w. transm oil cooler in main radiator as well).
fill it up, check level, and wait a minute or two to see if You can spot where it leaks.
Hope it helps.