On my laptop with macOS Sierra 10.12.5 (but it was happening with previous versions of the OS too), I get pinged by the system that I have only a few GBs left, I clean up, reboot, go from 4 GB left to 30 GB left, and within a couple of days, not doing anything special (not downloading movies or anything space-hungry), I get told again I have only 4 GB left ... I went through that cycle a few times now, and I'm wondering what could be causing that behavior, and how I could stop it. Any idea or pointers to what I should look at on my HDD to try and understand this? (sleep images, sparse bundles...)
Note: this question is not about using a disk inventory app to find the large files that take up too much space on my HDD. What I am interested in is why I got from 30 GB after boot to 4 GB without installing anything on my Mac. I want to understand which process is doing that and stop it.