Most likely it's dripping back into that location from the thermostat gasket. It is very common for the thermostat to drop out of the groove during installation. If you tighten it down after this happens, it warps the crap out of the housing, and it won't seal no matter how much goop you shove in there. Check the mating surface of the thermostat housing with a straight edge. If it's warped, don't sweat it too hard, it's like a $12 part at any autoparts store.
Kurt