Found it!

I went digging to find the power for the post light in the front yard. I found it! It runs under/alongside the front walk. It might head into the house at the basement under the corner window in the kitchen, or maybe it runs under the walk and porch to appear in the wall by the front door of the living room.

Wire, shown against the shovel so you can see it. It was in a 1" rubber sleeve.

I figure they cut the wire while frantically making the house presentable for sale (you should see some of the slap-dash interior painting). They likely abandoned the switch (you're not supposed to do that) in the wall.

I'll run a new line out rather than trust this one, so there's no need to locate the other end. 14/3 UF-B is about $1/foot. Code says it only needs to be 12" deep as long as it has GFCI.