Hard drive space decreasing for no apparent reason

Have you emptied the trash lately?

Look for iOS backups…

/Users/[USERNAME]/Library/Application Support/MobileSync/Backup

OmniDiskSweeper shows you the files on your drive, largest to smallest, and lets you quickly Trash or open them.

https://www.omnigroup.com/more/

Purging local backups

Please note that although this doesn't affect your remote backup from Time Machine, this will get rid of the redundancy (at least until the next Time Machine backup) that a local backup disk will provide. If you need such redundancy or are worried about the recovery of your data then you would be best served to let macOS determine when to purge these files.

Start Terminal from spotlight.

At the terminal type tmutil listlocalsnapshotdates. 

Hit enter.

Here, you'll now see a list of all of the locally stored Time Machine backup snapshots stored on your disk.

Next you can remove the snapshots based on their date. I prefer to delete them one at at time. Once my "System" disk usage is at an acceptable level, I stop deleting but you can delete all of them if you want to reclaim all of the disk space.

Back at the terminal, type tmutil deletelocalsnapshots YYYY-MM-DD-HHMMSS , where will be one of the dates from your backup. This will be in the form of xxx-yy-zz-abcdef. Try to start with the oldest snapshot.

Hit enter.

Repeat for as many snapshot dates as required

http://www.thagomizer.com/blog/2018/03/27/cleaning-up-time-machine-local-snapshots.html

tmutil deletelocalsnapshots /  # deletes all the snapshots

Also see if /private/var/log/asl/Logs

/Users/YourUserName/Library/Logs

Are containing huge number if files.

Where are you getting this information from, is it About This Mac> Storage, if so the information in there is often wrong.

Try rebuilding the Spotlight index,

How to rebuild the Spotlight index on your Mac – Apple Support

Move the Macintosh HD (or the name you gave your disk) in to the Privacy panel.

Quit System Preferences.

Open System Preferences> Spotlight> Privacy highlight Macintosh HD and press the minus button.

The mac will start rebuilding the Spotlight index.

Check by clicking on Spotlight in the menubar and enter a word, if it is indexing you should see a progress bar.

You can also use the Terminal app in your Applications> Utilities folder to force Spotlight to reindex.

Open Terminal.

Enter this command,

sudo mdutil -E <drag the icon of the drive you want to reindex here>

(make sure there is a space after the E.)

enter your Password.

(what you type will not be repeated on screen)

press Return

Terminal will confirm if indexing is enabled on the Volume/ drive and after a few seconds 

Spotlight should begin reindexing.

To confirm click on the Spotlight icon in the menubar, type in a word, if there is a progress bar, Spotlight is indexing.

Omnidisksweeper will give an accurate account of the storage used. When the app has created its overview

you can look at the Users folder in the output and see what each user has stored, you can then delete files from there.

Do not delete any files or folders in any System or Library folders or any files you do not understand.

OmniDiskSweeper can't see some items Apple hides in recent OS versions, no idea what Apple is thinking. :(

Safe Boot, (holding Shift key down at startup), does the problem occur in Safe Mode? Could take 10 minutes.

Safe mode attempts to repair Disks & clears lots of caches & loads safe Drivers, & prevents loading of 3rd party extensions, so if Safe Mode works try again in regular boot.

EtreCheck is a FREE simple little diagnostic tool to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac. It will not display any personal info.

https://www.etrecheck.com/

Pastebin is a good place to paste the whole report if you capture the URL while there…

https://pastebin.com/

Whew, they've changed pastebin & made it harder, but after pasting in, click Create new paste button, then Embed button, then copy the URL...

<script src="https://pastebin.com/embed_js/KuvnghqA"></script>

The important part of the above is...

https://pastebin.com/embed_js/KuvnghqA

Workable but harder for me to work with...the Note tool on the bottom of this editor's toolbar, as shown in the image, to copy and paste the output from EtreCheck. In a Reply before you click post, look for this to add longer texts...

OK, anxious to hear what you find.