A few hours later, I get a message from him, asking if I took my cup back, and telling me that it's not on the balcony anymore. I think he's joking and that he's taken it himself, but he convinces me that he's serious, and that some children were playing outside his window and may be responsible. Cups are expensive, so I go down to try and see if there's any chance of recovery.
So he and I put on coats and go out. We see a couple of 8-10 year old children playing about 20 feet off, and I'm already laughing at how this is going to go: "Hey, did you take something from that balcony? Well it's a ladies' menstrual product. It goes in my vagina, please give it back." Fortunately, we found the cup on the ground a few meters from the balcony, and no hilarious conversations with children were necessary. He wanted to go talk to the kids about it, since stealing people's stuff isn't cool and there was no way the cup just walked itself over there, but I was just happy to have the damn thing back.
So beware when sunning on the first floor: cups are clear, squishy, and look fun to play with.