On my Mac running macOS Catalina, I noticed there were much less and less free space during past several weeks in which I had done little file additions or software installations. Scanning with Daisydisk returns with results as above, with a "still hidden" part of space taking more than 40 GB.
tmutil listsnapshots / and
tmutil listsnapshots /System/Volume/Data and found there were not a single local snapshot on my machine.
I've looked up about this question on AskDifferent before posting, and unfortunately none of the replies in theses two questions, Gigabytes of disk space shown as hidden space in DaisyDisk and MacBook HD “Other” all of a sudden almost full, solved my problem here.
Does anyone know what caused this "still hidden" space and how may I free it?