2000 Pontiac Sunfire leaking Coolant

My I left my car running yesterday, while I was inside my nieces house. It was running for about 10 minutes, I noticed when I waled out that my car was leaking coolant. After the car cooled down I checked the coolant in the reservoir tank and it was fine. I took it to a mechanic that checked and stated that he didn't see anything. He stated that maybe the car got to hot and for some reason that caused the coolant to leak from the overflow hose. The coolant was leaking on the right side in front of the passenger tire. Later on that day he told me to let the car run for 10 minutes to see if the same thing would happen and it didn't. I hope this is nothing major.