The '03s also have the problem with the intake gaskets. If you're losing coolant, I'd expect that to show up in a drop in the expansion tank and/or the radiator, regardless of where it is leaking.
As PDad mentioned, and you checked, the most common cause of a coolant smell inside the cabin is the heater core. Other than that, something toward the rear of the engine compartment where the smell could be pulled in the fresh air intake in the cowl is another possibility. Intake leaks, coolant elbow, heater hoses, and water pump leaks could fall into that category.
Ten Gigahertz had a similar situation where he was trying to track down a coolant smell a year or two ago. He never did find the source. Recently, he had to have the intakes done. It'* hard to tell if the two would be related, however.
Black 2000 SSEi, Silverstar Headlights, Angel Eyes fogs, 3rd brake light overlay, hi-flo cat, drilled 180 degree thermostat, Intense FWI, PCM, shift kit, and 3.4 Pulley. Solid front mount.