Never had this happen before; but I'm getting a little oil seepage around the second bolt back on the driver's side on this engine (the one hidden behind the temp sender boss in the picture). Long ago, I learned to always add Permatex to the middle two so oil didn't seep. I don't have a loose head handy to check; but is this a common thing too?
there are a few other holes that go through....looks like you found one of them. Just pull it out and put some sealer on it and put it back in, should be ok.
Had the same problem with a older small block Chevy intake manifold I had .I had to seal it up with thread sealer