Well found it.

There are three freeze plugs on the top of the VG33e engine. Seems for some unknown reason the middle one on my Xterra sprung a leak.

Looking at it after it was out and you could just barely make out where the coolant was sneaking by.

Looking at a similar symptom, and I don't have a Haynes manual. Are these in a location you could describe so I can see if I'm looking at the same issue? My water pump and hoses are all ~20k old, but Ive added well over a gallon of coolant and water in the last 4 days and wherever it's going it's not anywhere I can find.

They are on the top of the engine block between the cylinders. There are two of them. You can not really see them if you are looking in from the front.

You need to take off the collector to see them well.

Great - I've got a weird coolant smell with nothing visible; I wonder if this might be where it is