pontiac g6 leaking coolant from overflow tank from hole under tank that seems like

[Bull]

it belongs there? i was losing hot air from the heat when car was parked or stopped at light, i noticed tank was empty so i filled to cold fill line with recomended dex cool and took for ride about 4 hours later i checked again and it was well under cold fill line so i refilled only accidentally about 3/4 of inch over cold fill line..now i have coolant leaking from a hole (that seems to belong there)under my over flow tank when running, and coolant is just above cold fill line...im not sure if car has leaked before this but i dont think it has...could this just be from over filling or is that hole leaking indication of somthing serious??? got 4 kids and almost x mas trying to trouble shoot with some help..lol