When an email is processed by retention tag,it does stamp couple of properties on the folders as well as emails,some of the properties is PR_POLICY_TAG & PR_RETENTION_PERIOD which contains the GUID of retention tag (These properties depend upon the type of tag and action set on the tag).
But when the policy is removed from the mailbox,the PR_POLICY_TAG and some of other properties are not removed,even thou messages in Outlook doesn't show they are tagged.
There have been cases where we do see that these emails continue to be processed by the Folder assistant after policy is removed.This behavior has been documented in the Technet.
When a retention tag is removed from the retention policy applied to a mailbox, the tag is no longer available to the user and can't be applied to items in the mailbox.
Existing items that have been stamped with that tag continue to be processed by the Managed Folder Assistant based on those settings and any retention action specified in the tag is applied to those messages.
However, if you delete the tag, the tag definition stored in Active Directory is removed. This causes the Managed Folder Assistant to process all items in a mailbox and restamp the ones that have the removed tag applied. Depending on the number of mailboxes and messages, this process may significantly consume resources on all Mailbox servers that contain mailboxes with retention policies that include the removed tag.
If a retention tag is removed from a retention policy, any existing mailbox items with the tag applied will continue to expire based on the tag's settings. To prevent the tag's settings from being applied to any items, you should delete the tag. Deleting a tag removes it from any retention policies in which it's included.
In most cases deleting the tags as mentioned above might not be possible.If the policy is removed from a user and you are not planning to assign any other policy to this mailbox.We can either ;
1) Create a new policy with the disable Default Policy tag. OR
2) Put the mailbox on Retention hold.
Hope this helps.