Mac Hard Drive Storage Decreases even without downloading anything
Could be anything. App updates, new apps, a virus that downloads stuff for you, automatically installed software, or a host of other things. 

If you did have an application on your Mac like Dropbox or Google Drive, (either of which could automatically be downloading stuff onto your computer) it could be that, but you do not.

Here's what you should do: download an antivirus app (like BitDefender or Avast). Run a full system scan in whichever you download and make sure no viruses come up, as some malicious software could be doing this. 

Then try downloading Grand Perspective, which shows you everything that's taking up space on your mac. Run it through your entire hard drive. See what comes up, and save your scan; then try scanning again when your available space decreases again.

Please report everything you find out in a comment below so I can further assist you!