Okay, a mystery female flower has appeared. With these squashlike flowers, the females have a mini fruit underneath, shaped like the fruit to come. This narrows things down a bit. Round and smooth. Hmmm. Some kind of melon, maybe? I tried planting Sugar Baby watermelons earlier, but that was last year. And the mystery plants are turning up in way more places. I still think it’s something that got composted. Now I’m thinking cantaloupe.