I'm going with BS on the porous oil pan theory. If that were true, you'd be seeing an oil stain that wouldn't wipe off.
FYI - And feel free to correct me if I'm somehow wrong here - it's damn near impossible for a DIYer to prevent oil spillage due to the filter location. There's no way to control the flow out of the oil filter port hole.
Actually...just saw you have an EX. I don't know the filter location on those, so excuse me if I'm wrong there. The SI is a bear.
Ok, so I'm re-reading your post. You said you didn't find a puddle on your garage floor but you "saw oil appear". Well, where did it appear? On the floor? On the oil pan (that would fit in with a poorly cast pan)?
If it's just dissapearing from your dipstick, I'd say consumption
Joe