[ccmovrwc]

OoTP doesn't specifically say the locket was thrown out (which would be Kreacher), it only implied it (which would be Mundungus.)

My vote is on Mundungus. Since Harry now owns Kreacher, he would be able to simply order Kreacher to return the locket and that would just be too easy.

[Not A Snowbird]

If he puts two and two together and remembers the locket...