Yes, a few times over the last forty years...........just a single seed, fully developed and ripe with no chance of any hermies or males in my garden.
Only thing I can put it down to is "immaculate conception" or some random stray pollen from who knows where.
Apparently, male pollen can be carried on the wind for several hundred miles.
If a neighbour had a male plant, you would think that there would be many more seeds, not just a single one.