Okay folks, I'm about ready to cry....and that's not pretty!

About a week ago I bought an '01 SLK320 with 68K miles. No leaks anywhere -- of this I'm sure -- haven't seen a drop of anything in the week I've owned it. The selling dealer (a Porsche dealer) did an oil change recently -- pretty sure it's Mobil 1 0w40.
Last night, we got one of our rare cold snaps. About 32F/0C -- yes, that's what passes for "cold" down here in south Texas. Anyhow, this afternoon I back the car up the driveway...and what do I see but a good-sized oil spot! About where the rear main seal would be.
So, what's up here? Is it the cool weather? And yes, this is about as cold as it gets down here, so I can probably live with it if it only happens on cold nights. After all, I just bought the car so I'm feeling a little financial pain right now.
Words of wisdom? Suggestions?