What’s wrong with cukes?
That does not look like a disease to me. The cuts are too clean, and there are no precursor symptoms for those holes caused by diseases. If it was a pathogen, the leaf would first develop spots, which could enlarge and dry out and then fall through. Usually, this leaves a ragged appearance with dried crispy edges to the holes. Also, there would most likely be other spots that were not yet dried out or holey. Viruses, including mosaic, do not cause holes.
The most likely culprit is cucumber beetle. But, it is odd to not find the beetle. They are usually quite active.