While exploring, I found a set piece in the ocean. It was composed of a reed raft and two items with no icon. These items are in the third slot in my toolbar. The two items produced the quote "Oh the science I could get up to with this!" from Wickerbottom. I have not discovered any use for the mystery item in question. No mods were in use at the time.
The only step involved is finding the set piece that spawned the items. As I said, it was out in the ocean, far from any land.