Silly question but is there trails of slime? I have a Butternut Squash plant and mine is getting eaten too, but there is NOTHING to suggest caterpillars/slugs/snails. As a few of my plants have been being attacked to the point that there is nothing left (Broccoli, strawberries, and a few others too) all the same with no evidence what so ever of anything slug/snail related, I put out a plea on Facebook and someone suggested Pigeons. I do have a few pigeons in the local area as well as other birds as well, so my guess is that this is what is destorying my leaves. I have this weekend put up some netting but its too soon to tell if this is whats destroying my leaves.
Hopefully this helps, but this my first year growing stuff so I have no clue myself. x