First of all, it's not good to just "fill the tank to the brim." When you overfill the tank past its limit, your just taking out the ingrediants and addatives in the coolant that enable it to cool your engine. I had a similar problem with my car in the summer. About 2 times a month, I would go out to my car and there would be a massive pool of coolant on the ground. For some reason, it just drained all of the coolant out in about 3 hours. 2 months later, I figured out it was my waterpump going. Does your engine bay look like this?? This was the result of a broken water pump spewing coolant all over the fan, thus spraying it everywhere around the engine bay.