Well, there are only a few possibilities. A shot seal, a misinstalled seal, a leaking valve cover gasket (usually due to old seals or not applying RTV where the #1 cam caps mate with the head) or a leak between the head and the block (the area I mentioned previously).
Diagnosing is obvious enough, pull the cam gears and the upper, rear timing belt cover, check for oil. If it's up as high as the cam shafts or the valve covers I'd just go ahead and replace the seals AND the valve cover gaskets.. they're cheap and if they're stock (which sadly you can't really test for) they should be replaced.
If there is some oil along the bottom of the plate, but not higher it's likely to be the union of head/lower, rear timing plate and block. This can be a little harder to diagnose. I'd clean the hell out of it, put the cam gears and timing belt back on (but leave the upper front timing belt cover off), run for a few minutes at normal operating temps and then check for any new oil... use UV additive if you like. If you find no new oil take it for a short drive.
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