For thoroughness, I also search through 5000 of my last FB received (using F3 keyword search) to see if any of the buyers (which I would be willing to swear on what's now left of the aforementioned chocolate that I did actually have for this particular item) had left feedback in case I could trace it that way.

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Did you do the same search in the feedback you LEFT for others?  I find this much more effective because not everyone leaves it for me, and sod's law almost guarantees that the item you didn't get feedback for would be the one you're looking for.  This all assumes that you don't wait to get feedback before you leave it.