Currently IPAM shows the last response of the DHCP scope for reserved IPs within a subnet, not the last response of the IP itself. This is not very useful for identifying or cleaning up stale records.
In our environment we have well over 4,000 reserved IP addresses. As devices get replace or decommissioned those MAC address reservations do not always get cleaned up, and as it stands looking in IPAM they appear to still be active on the network because of the last response date getting populated with the last DHCP scope response date. For example we had a remote temporary training room setup with its own subnet, the WAN link was torn down a month ago but the IP addresses of the training workstations that were on that subnet show today's date for the last response date since they had reservations.
Reference Case # 523683