Welcome to the club!!!!
I would still take your car back to the dealer, but you might want to check out the link below and the subsequent jump after this link to read up on some other members that have complained of a coolant leak. Although you have said there is no noticeable leak, the coolant has to be going somewhere. Of course there could be a fault in the sensor or as the service tech suggested, air in the system, but a trip to the dealer is in order.
Coolant leak?
Good luck and let us know how things turn out.