can't find leaking coolant

I have a 1993 GA 2.3L that recently had it's radiator replaced.

A week after the installation i noticed a huge puddle of coolant in the driveway after a sub-zero, cold night. I filled it back up (around 2 quarts) and called the mechanic who replaced the radiator. I had to wait three days to get it in. I drove cautiously for those few days and didn't notice any problems until the morning I brought it back to the mechanic (another near 0 degree night). The mechanic pressurized the system and it held pressure, but as he was checking the radiator connections he noticed the petcock was a little loose (about 1/2 a turn) he tightened it up and I went on my way. That was 3 days ago. Yestderday there was about a tablespoon of coolant that leaked (coincidentally, just below the petcock), but you couldn't detect where it came from--all hoses, and conncetions were dry and the petcock seems tight.

Could there be a defect in the petcock?

Has anyone experienced a similar phenomenom?

found the leak...the upper radiator house was a little loose...tightened it up and seems okay.

congrats. My wife took my GA to a mechanic cuz I was out of town for a coolant leak. It ended up being a plastic water manifold coming out of the engine on the driver's side. When I got home, the car was still leaking. I checked it out and found the mechanic had reused o-rings on the manifold. These o-rings had ripped and water was leaking through. I called the mechanic and told him about it. He said it wasn't him, he had used new o-rings. So now I say DON'T USE MAJESTIC MOTORS repair shop in Tempe, AZ.