In SharePoint, when an Information Management Policy is executed on an item and Expiration is enabled, and that item is deemed to be expired, then the item will not be evaluated for Expiration again at the next evaluation time.
When an item is expired, SharePoint sets a value to the following fields of the item:
_dlc_Exempt – This is set to 1
_dlc_ExpireDateSaved – A timestamp – appears to be when the item was expired
There is also the field _dlc_ExpireDate, but this does not seem to be set.
To “unexpire” an item, so that it will be processed again by the next Expiration evaluation, clear out the value of the above fields.