Hi,
According to me the unplanned depreciaiton is posted in Assets by executing ABAA T-Code and then to book the same in FI the effect is done by executing the AFAB T-Code. In AFAB T-Code by selecting 'Planned posting run' and if the period is already posted then you can execute the same in 'Repeat'Run mode. Also consider that if nothing is done then the depreciaiton perios which is due to be posted, in such depreciation period the unplanned depreciation will be posted.
Regards,
Malhar.