Hard Drive fills back up with photos despite moving Photos Library to external drive

I followed this guide and moved my Photos Library to an External Drive, deleting the original Photos Library from the Macintosh HD. I gained 40GB of space and the Macintosh HD showed 102GB free space in total. Then over the next 10mins the 'Manage Storage' screen showed the Mac filling up with 'Photos' (I wasn't doing anything) - first 7GB of photos, then 18GB, then 32GB, finally 37GB. At the same time the Machintosh HD returned to 65GB free instead of the 102GB free it had a few minutes earlier.

There is no Photos Library on the Macintosh HD that I can find - only the System one on the External Drive. A similar question was asked previously on this article but because the user wanted to upgrade the operating system, the help offered was focussed on finding space. The main use of space in my situation, aside from 'other users' (54GB) is the photos on my account (40GB). How do I find those photos to remove them? Again, when I deleted the old Photos Library, the 'photos' listing on the Manage Storage screen went from 40GB to less than 1GB. Then it refilled to basically the same level. Why?