If I understood your question correctly then my answer is as below :
Here I assumed that all eNodeBs of the network have X2 connection with HO enabled.
As UE moves away from serving eNodeB/cell, radio signal strength gets weak. UE reports to eNodeB by sending event A2 for the serving cell. If UE is unable to detect and report possible target cell(s) and serving cell is not able to deliver data due to poor radio channel condition, source cell/eNodeB send Error Indication (say failure in radio interface) to MME.
MME may initiate detach procedure towards the UE.
Whatever I explained above is an example when UE is moving out of coverage of a serving network. In case when UE is moving out of coverage then only UE would not be able to report any target cell/eNodeB .