pls91fc

Hello, I have a 91 n/a and I have had my car in the garage since beginning of november b/c I live in New England. I have been starting it up just about every week and letting it run for 15-20 mins just to get everything moving. The other day when I was moving it around the driveway I noticed it was leaking something. So i put a pan under it and come to find out its coolant. When it was running I looked to see if it was leaking but I didnt see anything. I left the pan and it collected a little bit of coolant, maybe 1/2 of an ounce overnight. It doesnt seem to leak when the car is running just when its sitting. I looked at the service manual to see what could possibly be goin wrong but havent looked much at the car yet. Any ideas, anyone seen this b4? other than the leak car is running great. Thanks for any help