Finally started thinking about the top of the engine, and discovered this gem on Sunday... there's a hole in the top of the intake manifold that was covered up with a piece of tape. Not sure how I didn't notice it before, but in my defense I pulled off the manifold and put it in a box with the hardware when I first got the engine in my basement, and haven't thought about it since.
You can see the tape covering the hole in this photo from the middle of March.
Up close:
I spent a while reading up on how others have dealt with this issue (holes and/or cracks), and the material is glass-fiber reinforced nylon (PA6-GF30). There's some plastic epoxy that you can schmoo together and build a plug to seal it up. Or, you can buy a nylon spatula, and cut out a patch to either epoxy or melt (with a sufficiently hot soldering iron) into place. The manifold is currently soaking in a big container with lots of degreaser.