Look - my wall, pre GMEP
The first hole, at ceiling level:
This is an interesting hole in the top plate. It looks like someone drilled to run a wire and changed their minds. We're going to fill it in.
Here's the bottom hole. I made it larger because we've got some other work to do in this hole in the future. I'll only be closing it up temporarily for now.
This is the hole we think is responsible. It's the power run to the outside light by the front door.
This is SO much better than the things I'd been fearing. We weren't initially aware of the power cord into the external wall because we hadn't opened up the wall high enough to see that it was there. Bob spotted it looking down from above. So now there needs to be a trip to Home Depot for some fire stop foam suitable for wire holes.