Losing Disk Space For No Reason!

You can use programs like GrandPerspective or OmniDiskSweeper to more easily see what is using your space

 

Try running onyx. It could be cache or something similar.

 

check your logs folder in the library, could be they need cleaning out

 

Have you tried restarting? Very likely, a good amount of that space is being used by the swap file. Restarting will clear the swap file... if you want to see how big the swap file is, look under System Memory in Activity Monitor.

Also, I'm a fan of DaisyDisk for seeing what space is used where (though it's paid, unlike the other two mentioned earlier). I've found it to be much, much faster than GrandPerspective or OmniDiskSweeper. OmniDiskSweeper gets confused by TimeMachine backups. I once had it tell me I had over 1 TB of information on my 250 GB external TM drive.

 

You said you emptied the Safari cache, but did you clear the webpage preview cache at:

~/library/Caches/com.apple.Safari/Webpage Previews/

That thing gets huge if you visit lots of websites daily.

 

Check to see if you are using FileVault.