I am kind of an old monkey wrencher and gear head, so while I don't have any specific on a G6 with this problem I am going to toss out some common / obvious considerations:
1) did it actually get filled fully? (i.e. did only 4 quarts get added, and after running and priming the filter it is now a quart low in the pan / dipstick)
2) is there another path of leak you haven't found? ( i.e. is the coolant reservior full of oil, etc) - highly unlikely as these cars are high pressure so not gonna happen, but you get the point of the question, look for the oil somewhere else, it doesn't just disappear. either smoke, ground, or another location onboard.
3) how sure are you of no leak? It is common with these alu block, alu pan engines to get a bad seal to your filter unless you crank it down tighter than hell. I ran a quart out without any sign of it because I never let my car sit and idle - it was only blowing past the filter with the engine running, so for me that was going down the road - cranked the filter tight and no more leak.
What I would do if it was me at this point:
Top it up to full, elevate the car up enough to get under it while level (so 4 stands, not 2 ramps under the nose), with the engine running using a towel flashlight and mirror go over all the spots it normally leaks and inspect again - be careful do not get the towel caught in the fan or belts :P
check the filter, the drain plug, as much of the pan to block gasket as you can, as much of the heads to block gaskets as you can, etc.... Not that many places it can actually get out.
Check the exhaust at initial start, idle, and with a little rpms to it.
Check every fluid for discoloration, coolant colored like carmel, transmission more tawny brown than burnt, power steering fluid, brake fluid, wiper, etc - all of em... you probably wont find it here, but be nice if it was an easy find like in the coolant.
My 06 3.9 engine typically burns about 3/4 of a quart every change (3k miles) - and I am at 70k... so I'd expect you are burning a little, typical high mileage engine, combine that with some other issue and it can be harder to find.