when the car is cold start it up and pop the radiator cap (it won't spray as the engine is cold)...look for bubbles in the coolant to see if you have a headgasket leak...you could have a very small one and burning a little bit of antifreeze (enough that you won't see smoke)

also i'd put the car on a clean surface and start it up and let it get warm ~ about 5 minutes should do (turn on a/c etc to make it run hard to it gets warmer than usual idle)...move the car and look for leaks on the ground