In this situation the VLR performs an implicit IMSI detach.
This procedure is used to update the location of the Mobile Station in the network and is described in more detail here.
The HLR is not informed of this and the VLR does not acknowledge the MS about the IMSI detach.
If the radio link quality is poor when IMSI detach occurs, the VLR may not properly receive the IMSI-detach request.
The IMSI detach procedure informs the network that the Mobile Station is switched off or is unreachable. Once assigned, the MS sends an IMSI detach message to the VLR.