Removing a user from a group (or deleting the group) for which the Read permission is granted on a folder synchronized for the given user is not detected as a deletion by Drive

Because these events are not detected by the audit change finder.
There are some events raised: "group_modified" and "group_deleted" though this is not enough to track that a user has been removed from the group ot that a removed group was one of the user's groups, and what synchronization roots it could impact

The simplest seems to have a Drive listener on these events and force the clients to make a remote full scan in this case. Though optimizing at least to impact only the users related to the group changes would be nice.
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Finally, we are now relying on the NuxeoDriveGroupUpdateListener to handle the group change events fired by UserManager#notifyGroupChanged(String groupName, String eventId, List<String> ancestorGroupNames) and make sure the FileSystemChangeFinder detects them.
The tracked events are "group_created", "group_deleted" and "group_modified".

For SQL-base directories, the notifyGroupChanged method is called directly when invoking createGroup, deleteGroup or updateGroup on the UserManager.
For LDAP-based directories, it must be called manually, through an Automation operation or a Seam component for instance. In this case, note that when deleting a group, the list of group ancestors must be computed before the group is actually deleted to be able to provide the ancestorGroupNames parameter to UserManager#notifyGroupChanged(String groupName, String eventId, List<String> ancestorGroupNames).