Please help to locate oil leak

[Tn52]

Hi everyone:
For the last few months, after driving my X5 for about 30 min, i can smell an oil burning odor somewhere under the hood... From searching and readings, many posts mentioned about leaks around the valve cover gasket so I tried to look for signs of leak around the valve cover but I did not find any. In fact, I used paper towel to check around the valve cover' front, back, and sides but they are clean and bone dry. I then looked under the car, passenger side, and saw a wide patch of dry oil burn residues on the underside cover of one of the catalytic converters (the inside one - closer to the engine). From the catalytic looking directly up, everything is clean and dry from this side of the engine, the 2 x 3 into 1 headers are also dry. Assuming an oil leak should drip down from a location above it due to gravity, where could it came from? I concern that this problem could be a major repair...
2005 X5 3.0: Auto.