A modification of Privateer's comment: Humans can recognize traps being part of a trap structure (such as a trap in a trap fence, or a line of traps), but not when they're alone. I had a stupid visitor (as in not invited, but not hostile either: I don't tend to chase them away) at my homestead who first cut down the trees surrounding a trap, and then get his legs broken in that trap, as it was no longer recognized by him. I've also found a dead merchant in a stand alone bear trap of mine. While both incidents happened a couple of years ago, I don't think that part of the NPC logic has changed since then.