Leaking antifreeze

Just need to see my mind straight here. I noticed sometimes I have a leak under the car near the oil pan, after driving the car for awhile then parking it. I thought it was a oil leak given where it is under the car. I looked at the engine and did not see any leaks. Last time I noticed it I pulled the car ahead and found it to be coolant.
Ok I first thought frost plug, when the car gets hot and pressure builds it leaks...sounds right...but shouldn't it leak all the time. When the car is cool nothing at all.
If it is a plug I guess there are some behind the bell housing? {4 spped car} That seems to be the only place that would leave the puddle by the oil pan.
I figured with heat they would expand and leak when cool.
AL