Marking traps as player’s could be done in a similar way as seen traps.
Marking vehicles as used as the player could be done by storing “last controlled by” ID in the vehicle, to verify that current character is the one who set it up.
But then, if we consider all kills performed this way to be done by the player, we’ll also count zombie children and innocent NPCs as player’s kills. That is, if you left your armed mine collection in an orphanage, you would feel bad about the children.
Unless we only count it for the purposes of killcount.