GHG
I did have a leak from one of the rocker covers and although it ended up wetting the fins on the cylinder head rather than on the top of the cylinder, oil is pretty sneaky stuff so it is entirely possible that a leak there could be causing the problem.
Oddly (IMO) the bolts that pass thru the rocker covers don't directly provide the clamping force that holds them in place.
Instead, the bolts are shoulder bolts that bottom out on the top of the head and they provide the squeeze for the rubber washers under the bolt heads.
It is the rubber washers that provide the clamping force to the covers that results in the covers pinch on the gaskets.
Over time, these rubber washers seem to deform and take a 'set' causing them to loose some of their clamping pressure. That's why I used some Permatex Ultra Black to seal my gaskets.