Test for coolant loss

I just bought a 2007 with 99K miles from a dealer. The car has a two month, 2000-mile drive train warranty from the dealer. I drove it on one 1000-mile trip so far and it runs great, 50+ mpg too, but it lost an entire reservoir of coolant, from "full" line to almost empty. I checked the level in the radiator before, during, and after the trip and it never dropped below completely full and looks very clean. I have the Carfax service history which says the ICE coolant pump was replaced at about 65K miles, and the coolant was changed then. I checked with the dealer and they did not perform any service to the coolant system while they had it, other than a level check.

I did a visual check, including taking off the left front fender skirt and checking at the thermos tank, CHRS, and the three-way valve. I could check the complete radiator, most hoses and connections, and the some of the engine block which was nicely cleaned by the dealer, and could find no telltale pink residue. There were no drips on the ground during the trip or since. There is no moisture or odor in the cabin.

There is no sign of sludge on the dipstick or in what I could see of the valve cover. There was no visible excess vapor in the exhaust on some cold mornings recently.

Today I went around town trying to find a radiator pressure tester at the parts stores with tool loaner programs, and nobody locally has the right sized adapter for the Prius. One clerk said, "Is that a Toyota? We just don't see them in here." (Too reliable? Or nobody works on them?)

My questions are, 1) should I buy a cheap-o Mityvac tester for $45? If there's a minor head gasket problem, I want to find it while I have this warranty for another month. 2) Am I overly concerned about the head gasket? It's driving fine and everything is very clean. 3) Any other ideas about where and how else to look for a leak?

I want to trouble-shoot this myself to avoid charges for diagnostic time at the dealer, especially if the engine tests out fine and it's some stupid thing I didn't think about.

Thank you as always.

 

The heat works fine. There are no signs or symptoms of air in the system.

I think I'll go ahead and buy the radiator pressure tester, then do the compression test if needed. I have a Mini VCI. Thanks.

Another question--the manual for the pressure tester says I can remove spark plug wires one at a time to identify a leaking cylinder by watching pressure gauge needle fluctuations smooth out. I hope I don't get to that stage, but is this advisable with the Prius while running in inspection mode? And I assume you'd do it by disconnecting the primary of each coil?