2005 Nissan Altima Can't find Oil Leak

Recently i've been smelling oil while coming to a stop when i'm driving, especially when the heat is turned on. I've thoroughly monitored the ground and have found no signs of a leak (drip). So my assumption so far is that it's leaking onto the engine somewhere and is burning off before it reaches past the block. I've looked without taking the intake manifold off and didn't find much. I dont really notice a significant drop in the oil so it's not a big leak. Just enough to smeill. I would like to investigate or even change the valve cover gaskets and use process of elimination to see if its something simple and economic. Question 1, do you feel that a leak coming out of a valve cover would have this situation or do you feel I should probably go deeper in the motor and change the head gaskets, while looking at oil lines for cracks or leaks. Because right now I can't find the leak I can only smell it. Thank you for your time, I hope to hear back from you soon.