Coolant expelled from reserve tank

Hello,

Today my car did something I have never encountered. After being driven at highway speeds for about 30 minutes, the car was parked and turned off. About an hour later I found a puddle of coolant under the car. I traced the leak to the expansion/reserve tank. The top of the tank was wet with coolant. Has anyone else encountered this type of behavior? I have topped off the tank last weekend and may have been just slightly above the full mark with the engine cold (car wasn't driven until today). I am guessing I overfilled the tank (3-4mm?) and when the radiator cap purged the coolant into the tank it came out of the cap where the overflow hose from the radiator attaches. I would appreciate everyone's opinion on this occurence. Thanks in advance.

P.S.: The engine stays at normal temperature and shows no signs of problems.

when the car gets hot the coolant backflows into the reservoir when topped off at the radiator, thats normal. If you start the car and run it until it gets hot with the radiator cap off... as soon as the fans come on note the coolant level... then watch as it slowly goes down as it cools off, its quite a phenomenon but its normal.