1991 Chevy Corsica Coolant Leak

I recently purchased this car used and have discovered that I have a coolant leak that is difficult to pinpoint its origin. The car does not leak any coolant while it's running. I notice a small puddle after the car sits for a couple of hours. Recently I noticed a puddle on the ground and it appeared to be coming from the drivers side on the backside of the transmission pan area near the firewall. There is no oder of coolant in the car when using the heater and no coolant inside the car. I plan to put some cardboard under the car tonight when I get home to help assist in the location of the leak. I did take the car to a self service craft shop over the weekend and put my hand on all the heater hoses and around the water pump weep hole. I didn't find any leaks. I even let the car run for 20 minutes and there were no puddles on the ground. The car runs good and there is no mixture of oil/water in the crankcase. The oil is clean. The engine temperture does not appear to be running hot. The gauge reflects the temperature to be in the mid-range (halfway between the C and H.