Take off the inspection cover and get a look up inside to see if the oil is in there as well. It should be if it is a rear-main.
Otherwise, a rear oil pan leak (more common) may be the culprit. Once it runs down and air blows it around it'll go all over the place and it doesn't have to be a ton of oil.
'09 PSM G8 GT - OPG Intake, DOD-Delete w/ Stage 1C 218/229 .600 Lift 117.5LSA, Kooks 1 7/8 Mids, Solo Axle-Back w/ X-Pipe, SuperPro Bushings (Front and Rear). BMR Sway Bars (Red)
"If you want to criticize someone else's work, be sure that your own is in order first."