Physical objects often form a group such as objects in a shipping container. RFID enables us to identify each object and even the container itself. However, current RFID does not provide information on IDs missing from a group. This paper proposes a method to determine the unique IDs of objects missing from a group without any external database or verifier. The proposed method logically splits a group into mutually overlapped sub-groups and writes group-related information, which is generated from the unique IDs of objects in the subgroup, to RF tags' memory. When we check the integrity of a group of objects, unique IDs and group-related information of RF tags are extracted from RF tags' memory. With an iterative decoding over group-related information with the unique IDs of identified objects, missing IDs are determined. A numerical simulation reveals that the proposed method can identify 96-bit unique IDs of up to 64 objects missing from a group composed of 100 objects by writing 840-bit group-related information to each RF tag. We also examined the performance with an experiment and confirmed that we can successfully determine 16-bit IDs of up to 12 missing RF tags from a group of 20 RF tags by writing 280-bit group-related information to each RF tag. The experiment results agree well with the numerical simulation.