shot in the dark, but if you used anything other than the inteded bolt for the coil bracket you could have a crack in the valve cover and know about it...i had a valve cover crack on me once so it is possible
good luck man

I just don't know at this point. I inspected the valve cover pretty thoroughly last time it was off- didn't look real closely at the coilpack mount, but I guess I can remove the coilpack and inspect that in situ. I have personally never used a bolt other than the factory bolt in that location, though I did omit the lower bolt (the one that's impossible to reach) the last time I reinstalled the coil pack. I wonder if that decision has now bitten me on the ass.

Last night I noticed another odd thing. I wish I had a picture, but I'll try to describe this. Picture the area where the head meets the block. Specifically, the corner of the block on the exhaust side at the back. There's a little flat area maybe 1cm^2 where the block protrudes out and the head doesn't cover it. There's oil on that surface. Just a tiny bit, but it's there. What I'm trying to figure out is whether oil is leaking out there, or did oil run down there from above? I tried to clean everything pretty thoroughly last time the cover was off, but that area just can't be reached easily- the downpipe blocks access.

This is just aggravating me. At this point I'm thinking that maybe after the first of the year I'll pull the head and replace the gasket. With the timing belt off it'll give me a good excuse to pull the cams and replace all the lifters while I'm in there.