Furnace: pipe is leaking when switched to air-conditioning
That plug is where I add biocides to prevent mold buildup in the pan, mold is the main cause of obstructions , if the pan is starting to drip you probably have some cleaning to do as the water should not build up in the pan and a small layer has probably raised the water level because it is acting like a dam. 

I have had customers ask how can mold grow in there it’s only water and metal, well this is almost true but dust and fine organics get trapped in the coils and drip down to the pan, when the conditions are right a mold colony starts growing, 
For this reason pros recommend having your unit serviced, adding this biocide treatment is a requirement for large commercial systems to prevent legionnaires disease, I have not heard of it in home systems but it is possible. 
So you may need to clean and treat your pan or eventually the drain line will be totally blocked and the water may come out in other areas less noticed and cause problems.