How would you glance over making a trip wire? Just saying they are kneeling down and putting a string, which would be obvious regardless? Or not mentioning anything if the other character wasn't around? But lets put it this way. If I'm in a tournament and run through a doorway and the opponent says, "Oh yeah, and I had put down a trip wire there earlier but you just weren't around", wouldn't that be a little suspicious? Even if they did plan ahead that a trip wire was there. Plus a character could notice a wire in some cases so if they trip on something you didn't even tell them about beforehand, your character could have spotted a wire there so you wouldn't have said they ran through.