Is placing a piece of wood with a long nail under someone's car tire, without knowing whether it punctured the tire or not, a sin that necessitates expiation?

What you did is impermissible and you incur sin for it. Its expiation is regret and seeking forgiveness. If you know or strongly believe that the car owner was harmed, then you are responsible for compensating the damage. If you cannot find him, then give the value of the damage in charity on his behalf.