Why would coolant leak from the overflow bottle?

I have a 1996 Jimmy and recently replaced the thermostat. I know the raditor drain plug is not too far under the coolant recovery tank, if not that, you may simply have a hole in the tank. From what I've heard, I would use caution using stop leak products, but I am far from an expert. The question is--- is it leaking or just always low on fluid. A leak in the tank is possible but another possibility is the head gasket is allowing compustion pressure to get into the cooling system and blowing it out---check to see if there are bubbles inside the tank when the engine is running ---a sure sign the latter is happening. Jeep recovery tanks often have cracks and leak where the hoses atttach.