Hi,
please check following MS article:
https://learn.microsoft.com/en-us/answers/questions/470633/moniotr-content-search-using-sentinel.html
Unfortunatelly I was not able to find these in OfficeActivity log (especially I checked for SearchExportDownloaded event), as a workaround you could set-up a MCAS/OCAS alert and monitor these alerts in Sentinel.
Martin