PSA: ESRI REST API for Searching is Currently Limited to 10,000 Queries

As noted in the Response Properties section of the Search Article on the ArcGIS REST API Resource Page, there is a limit of 10,000 calls.

If you are using Admin Tools to search a large amount of content in your organization, and suspect that you missing a portion of the results, GEO Jobe recommends splitting up the search by the most relevant and clearly defined variable possible, (such as date created) with the 10,000 query results limit in mind.