Air conditioner

What has water that is higher than that ceiling?

Its not the toilet, shower or sink. Happens only in the heat of the summer when a/c runs all day. Plumber has put 12" x 12" hole in ceiling a water stain. PVC pipes and copper plumbing is visible. Water stain was linear and in an L shape. I feel it was the attic a/c unit. Drip pan was dry as a bone.

You might have to catch it when its actually getting wet. Assuming you can't see any signs of where water once was via that hole that would let you trace back.

Now let me get this straight... the water mark is on the 1st floor ceiling, but this is a two story structure with an attic mounted A/C unit?

Correct about the first floor ceiling. Has not dripped since hole was cut Aug16,2010, insulation was removed and about 2 cups of water came out with insulation. Copper piping and pvc visible but no water. Copper is for Rinai water heater,pvc is HVAC

Couple more questions. The leak then is in the area of the drain for the pan. Which is the PVC pipe you mention. This isn't a fairly new installation I assume as well.

Has there been any pictures hung, nails or screws driven in a wall the drain passes through in say the previous year before the leak was discovered?

Want to establish that the system has been leak free for a period previously to eliminate a faulty installation.