A bloom filter is used in Ceph to provide an efficient way of tracking whether an object is a member of a HitSet without having to individually store the access status of each object. It is probabilistic in nature, and although it can return false positives, it will never return a false negative. This means that when querying a bloom filter, it may report that an item is present when it is not, but it will never report that an item is not present when it is.
Ceph's use of bloom filters allows it to efficiently track the access to millions of objects without the overhead of storing every single access. In the event of a false positive, it could mean that an object is incorrectly promoted; however, the probability of this happening combined with the minimal impact is of little concern.