ARWARN [686] A re-indexing will be performed for each full text indexed field on the form at the specified time(s) in place of an update scan due to the absence of a last modified time field (field ID 6) on the form. [Lotar's Wiki]

AR System Message Catalog

ARWARN [686] A re-indexing will be performed for each full text indexed field on the form at the specified time(s) in place of an update scan due to the absence of a last modified time field (field ID 6) on the form.

Product:: AR System server
Description:: Server messages and notifications
Type:: Warning
Number:: 686

A full text indexing scan for changes was defined for the current form. The form does not have a Last Modified Time field that can support a scan for entries updated since the last scan. Therefore, each scan performed on this form completely re-indexes the form.
Investigate whether you can add support for a Last Modified Time field to the form to provide more efficient indexing.