How can I find out why I suddenly lost 50 GB of storage?

Consider installing a tool like Filelight. It gives you a visual representation of your disk usage and helps track down what's taking up space easily. It’s a straightforward way to pinpoint any large files or directories that you might overlook.

You might want to check if you accidentally sent files to the Trash. Sometimes, items can pile up there and consume a lot of space. It’s a good starting point and may help recover some of that lost space.

First things first, are you confused about memory versus storage? Check if you actually mean storage since Dolphin is a file manager and doesn't report on RAM. If that's the case, using commands like 'df' to check filesystem space and 'du -h' to list files by size can help you identify what's taking up your disk space. If there's a large discrepancy in the numbers from these commands, you might have some unlinked open files to investigate further.

I had a similar situation on Ubuntu where the /var/log/syslog file ballooned to several GB due to logging issues. You could use 'du -h' to investigate any unexpectedly large files or logs. Just be cautious; a large log file may signal a deeper issue that needs addressing.