Could have been dropped by mistake. Remember he has abducted and so presumably is carrying and subduing Alesha.Thew court decided, with no evidence whatsoever, that the knife was used in the abduction. However this cunning killer who thinks he's so clever just placed the knife on the beach. You'd think, what with his super human weight lifting strength, that he would lob that thing to kingdom come. No, just placed it on the beach ready to be found.
Possibly he left it to be found as a decoy as did not anticipate getting caught and matching it to a set from his house.
Perhaps he left it there another time