I have reached out to a few folks here at Tableau to see if they can help clarify the meaning of the null values in those locations.
One thing that would help is to share the version of Tableau were you using here and the datasource type?
I don't know how exactly to tie those back to specific sheets, or if that's possible via the workbook.
You could review the content of the query and deduce what axis, filter, calculation, etc it was meant to satisfy, but that could be quite laborious given a large and complex workbook.
To simplify the workbook you could copy/paste dashboards/sheets to their own individual workbook to reduce the scope of guess work if you suspected a few likely candidate worksheets.
The performance recording feature can create more questions than answers, it's value comes from what it does make clear. It's a work in progress.
From my experience such queries are those
tied to the common elements for several views
on a dashboard, such as common calculated
Quick Filters with Only Relevant Values flag.
Of course, your mileage may vary.
If their performance is a problem,
they should be traced to the datasource
(which is obvious -- just look at the code).
If you need to tie those queries in question
to the exact objects, then please look at the Timeline
(the upper view on a Performance dashboard)
to see where those queries reside among others.