Purge UE procedure is used by MME when UE becomes in-active for several days after detached from network. I mean to say that UE has already been detached but MME was maintaining UE context for few days(based on the configuration). Once timer gets expired, MME assumes UE will not come back to the network and it initiates Purge UE procedure to HSS.
Purge UE procedure is triggered when every things is cleared from the network for that UE. One more things eNodeB does not send detach since detach is NAS level message. eNodeB sends UE Context Release Request to MME for an UE based on the radio interface but detach is between UE and MME NAS level.