Starting with 4D v12 Release 3 (12.3) it is possible to see a new Warning message in the Verify log from the Maintenance and Security Center (MSC):

Index is flagged to be rebuilt

Here is what the message means:

When an index needs to be rebuilt a flag is set. If the database exits (normally or abnormally) before the index rebuild can be completed, the flag is unchanged; the index still needs to be rebuilt. The flag is only cleared when the index rebuild is complete.

Please Note: this is a Warning, not an Error. This message appears only to inform you this situation has occurred. No further steps are necessary, the index will be rebuilt when the database is launched.