If its a head gasket leak, the coolant will eventually take the O2 sensor out & you'll get a CEL + error code. Only took about 3-4 miles on my 4.0L Ranger after Ford had the Dealer replace the head gaskets trying to find a coolant sipping problem & I guess some coolant got into the exhaust when they opened that puppy up. Was driving it home on the freeway & the CEL turned on. Ford also got to replace both upstream O2 sensors too, on that warranty session.!!!

For it to use that much that quick & no external leak signs, it has to be a under pressure internal, or external leak & dripping, or spraying onto something hot, such that its instantly evaporating.& leaving no wet signs.
Adding a dye to the coolant, driving it some so it shows its used some, then checking with the UV light, you'd have a good chance of finding the leak. If its the water pump bearing, look around the front cover. If its a head gasket the spark plug internal insulator will glow yellow/green from the dye deposits & maybe since its using so much, the exhaust pipe might light up too.
Does it show any signs of running warmer that it should, if so, remove the radiator cap & sniff for exhaust, or idle the engine & look for exhaust bubbles in the coolant.
You could remove each spark plug one at a time & pressurize each cylinder with air & look for bubbles in the coolant & get an idea where the head gasket is leaking.
More thoughts for consideration, let us know how the cooling system pressure test turns out.


Let us know what you find.

My 05 was leaking from the head gasket, front passenger side. Although it was a small leak.