We see this sometimes when clients create "locallinks" in rich text editors. I think it can also happen with pickers. If the node being linked to is broken, you will get that warning in the logs. It is a little bit tricky to track down all of the culprits.

Here is a query you can run on the database to find the nodes that have the broken references. This query basically searches through all of the node data looking for references to 1393. I added some JOINs to cmsContentVersion so that you will only be searching on the latest node data. You can remove those if you think the query might be missing something.

SELECT TOP 1000  n.id
                ,n.[text]
                ,n.[path]
                ,pt.Alias
                ,pd.dataInt
                ,pd.dataDate
                ,pd.dataNvarchar
                ,pd.dataNText
                ,cv.VersionDate
FROM cmsPropertyData pd
INNER JOIN cmsPropertyType pt
    ON pt.id = pd.propertytypeid
INNER JOIN umbracoNode n
    ON n.id = pd.contentNodeId
INNER JOIN cmsContentVersion cv
    ON cv.VersionId = pd.versionId
INNER JOIN (SELECT cv_inner.ContentId
                  ,MAX(cv_inner.VersionDate) VersionDate
            FROM cmsContentVersion cv_inner
            GROUP BY cv_inner.ContentId) cv_top
    ON cv_top.ContentId = cv.ContentId AND (cv_top.VersionDate = cv.VersionDate)
WHERE pd.dataInt = 1393
   OR pd.dataNvarchar LIKE '%1393%'
   OR pd.dataNtext LIKE '%1393%'

Hope it helps!