Traps visible in PvP?
You are probably just a newb. XD
Unless there is a bug I’m not aware of, traps cannot be seen by other players.
However, enemy players have brains, and can use them to predict things, such as that a trap thief has already lay his traps on point. Also, they might actually see you place them by your animations. Since traps only appear at your feet, if you see a thief placing traps, well you know exactly where they are.
I hope you don’t take offense to my comment earlier, its just being a newbie comes with being new. Hope that helped.