Why is car overheating with no coolant loss and no thermostat problem?

two things to look at, #1, radiator cap is faulty, its a cheap fix, $6-7.00 at the auto parts store. The water pump may also be faulty, which will leak fluid from a seep hole under the pump housing. Open the hood, find the fan blade (Be sure car is not started) and the where the fan connects is the water pump. Grab the fan, and move it up and down. If it moves or leaks, then the water pump needs replaced. Also check the heater core to determine if fluid is leaking in the interior. Good Luck your thermostat may stuck in the closed position.