Can't find 68GB local backup folder (regular backups on an external hard drive)
It's likely your Local Time Machine Snapshots.

In Sierra, these are stored in /.MobileBackups/Computer, but it's not advisable to manipulate this folder directly. Either use the Time Machine application to delete individual snapshots, or disable local snapshots altogether by running the following Terminal command:

sudo tmutil disablelocal


I can't say why MacOS isn't cleaning them up after you've run an external Time Machine backup. EDIT: On my MacBook Pro, I also see some old backups that pre-date my last external backup.