Time Machine has a nifty way of continuing to keep copies of versions of files as you change them even when it can’t connect to a Time Machine backup volume. It creates local snapshots on your startup volume and other connected HFS+ volumes, and then later transfers these to a Time Machine destination when you reconnect on a network. A Macworld reader has questions about managing these snapshots.
I wrote about this last in 2015, after readers were trying to troubleshoot otherwise inexplicable missing storage on their Macs.