Perversely, the MacBook Pro has been going bonkers on creating system snapshots. To the extent of having doing 122 in the 9 hours I was away today. I do knows that Snapshots more than 24 hours old are deleted in High Sierra.
There were 127 local snapshots at 5 pm today. I turned off hourly backups in the Time Machine Preference pane then downloaded a current version of Time Machine Editor app, setting it the task of making backups every hour only.
Now there are only eight local snapshots in the system, those excessive ones generated every 30 seconds just up & vanished.
To check how many local snapshots are aboard, open Terminal & type "tmutil listlocalsnapshotdates” (minus punctuation) then hit the Return key. A list of active snapshots will be revealed in Year-Month-Day-24 hour time (in six digits).