I have only ever cleaned my engine bay 2 or 3 times in 3 years and I have NEVER had grime/oil/grease or anything anywhere around my valve covers. Come to think of it, I have never had it on any car I have owned except for the one time I had one with a leaking gasket.
OP, clean under your hood and then see if it comes back, if it comes back you have a minor leak. Re-torque your valve cover, it only takes a few minutes.