Something doesn't make sense here..
You fill it and it then maintains the level after it "equalizes" itself to the same level as before. Are you overfilling the tank? When the coolant expands it does purge any additional that it doesn't need. This could be part of the smell.
You won't see a UIM failure in the oil btw, that'* a myth which is spread by people that don't understand the coolant fills the cylinder and not the oil passages. Only minute traces might slip past the rings and then it would be burned off by the heat in the crankcase.
Your best preventative maintenance is like Dan suggests pulling the TB (3 nuts) to double check that the UIM isn't coolant filled inside. Just odd that it equalizes itself then stays put. Usually you'd see it disappear a few times, then hydrolock. The LIM holds about a half gallon.
00 Regal GS: All kinds of cool stuff.