Why would smoke come from behind the engine?

Bend over a little farther... doesn't that feel good to see the oil leaking leaking out of the valve cover? Probably not, but those are common for leaks, I think it may be a valve cover gasket. Get a neoprene one, not cork if you plan to change it your self. Could be leaking from anywhere back there I have seen them leak from the distributor. Check and see if you can spot the leak you may have to power wash the engine first if it is very dirty and greasy.

I agree, oil leaking from the valve cover gasket will seep down the engine and burn away when the engine is hot enough. That's why you will probably notice smoke coming from the engine (in your case the back).

I suspect that when you come to a stop in city traffic you see the smoke leaving through the grill or from under the car, and a small wind might even help it move into your cabin if the windows are open. Valve covers aren't that expensive to replace on older cars, but on newer cars labor can be more.