I assume things will be different for an extra drive, but for files on your normal drive things are even worse than you describe. Maybe this should have been a comment, but I think an answer stands out better in case someone reads this in the future:
Time Machine only keeps hourly backups for the last 24 hours, only all first daily backups for the last month, and only all first weekly backups until the disk is full.
So, even before Time Machine runs out of disk space, if you run Time Machine hourly then files that did not exist on your Mac for more than a day might be deleted from your backup after a day. Or, when running daily, files that did not exist on your Mac for more than a week might be deleted after a week (or a month; I never tested that).
For example:
- Run Time Machine at 10 am.
- Add some files to your Mac at 11 am.
- Run Time Machine at 1 pm. The files from step 2 will be added.
- Delete the files you created in step 2 from your Mac.
- Run Time Machine at 2 pm. The files from step 2, written to the backup in step 3, will still be on the backup, but no longer on your Mac.
- Run Time Machine at 11 am next day. All yesterday's backups will be deleted, except for the first which was created in step 1. Hence, your backup no longer holds the files you created in step 2 (and were deleted from your Mac in step 4).