What does Snape accuse Harry of stealing when they are in personal store cupboard to make polyjuice potion?

Snape accuses Harry of stealing the rare and extremely valuable powdered unicorn horn when they are in the personal store cupboard making Polyjuice Potion.

This accusation is part of Snape's ongoing antagonism towards Harry, and his suspicion that Harry is somehow connected to the theft of the horn, which was later revealed to be stolen by Draco Malfoy.