| Just been looking at the parts diagrams and if you're 100% sure the gasket isn't leaking, it looks possible it could come from the join of the cam journals?? The bolts that hold these down have a secondary thread on them for the covers to screw down onto. If one of them undid itself when the cover has been removed, it could be enough for a leak???? Especially when it warms up.
Long shot??..........they should be tighter than the cover screws but so are the pulley bolts on the 748/996 but they sometimes unscrew when the tensioner nut is undone. | 
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