Where is the AT Oil going?
 

Mercedes 126 722 auto transmission fully rebuilt 5K miles ago; now AT oil is disappearing at a rate of 1 quart per month; absolutely no leaks visible anywhere under the car; dry as a bone; no leaks visible from dipstick, new oil cooler lines dry as a bone; no leaks at front seal; no leaks at rear main seal; where is the oil going? what is the cause? what is the fix?

check engine coolant, may be a crack in the at cooler, which is in the radiator side tank. Would be pushing at fluid into the coolant, coolant will be looking like pepto bismol

Does your transmission have a Vacuum Modulator. The only time I had Trans. fluid disappearing like that was when my VM was leaking and the engine was burning the fluid through the intake.

It seems to me that it is most likely to be seeping by the internal diaphragm on the vacume module also,These are very inexpensive and easy to install.

+3 on vacuum modlator.
Easy to check by pulling off the vacuum hose at the modulator and seeing if there is trans fluid in it. Might be best to do this shortly after running the car.