External drive photos to wife's iCloud?

Strange Apple Photos behaviour.

I am completely baffled by something that has just happened with my wife's Photos library.

I am part-way through editing 2000 photos in Capture One of a recent trip. These photos are stored on an external hard drive that is connected to my Mac. Some of these photos include ones taken on my, and my wife's iPhones but they are largely SLR-type photos transferred to the hard drive via the cameras SD cards. I used Image Capture to transfer images from her iPhone into the folders setup on the Mac. None of the photos have been exported to the Photos app on the Mac and they do not appear in the Library of Photos.

My wife has her own iCloud account. I have my own iCloud account and the Mac that I am using to work on the photos uses my iCloud account login details. We use Family Sharing. We have not setup a shared Library in either her, or my Photos app. I do not subscribe to Capture One Live, which I understand allows collaboration when editing pictures.

This morning, my wife got a message on her phone telling her that her iCloud account is almost full. And indeed, after investigating, ALL of the photos of the recent trip, and only this trip, that are stored on the hard drive have been copied to her individual iCloud account and are present in her Photos app.

I cannot understand how this has happened as neither of us have exported the 2000 or so pictures from Capture One and it is a mystery as to how her, separate iCloud account has picked up these pictures. I would have thought that if a Photos app was to somehow receive the new pictures, it would have been my iCloud account; the one that is directly associated with the hard drive and Capture One.

Has anyone got any ideas as to how this has happened?