Contrary to @Ratslinger’s previous reply, I have found that the temporary Base report file is, in fact, saved on my iMac in a hidden folder path similar to this:
/Users/[username]/Library/Application Support/LibreOffice/4/user/temp/lu87681ce1ryv.tmp/
Within that folder, the file has a name similar to lu87681ce1rz0.tmp.
Today I discovered that my LibreOffice “user/temp/” folder contains more than 30 of these so-called temporary files, dating back to November 2017.
Each time I launch LibreOffice, it displays in an untitled startup window my Base database, several Calc spreadsheets, and all of the temporary reports that I created during my earlier LO sessions. I delete these reports there, but apparently I missed some in 2017, 2018, and 2019.
I suggest that LO should immediately delete each Base report file when the user closes it, since the report file cannot be opened again at a later time.