How do I close a password-protected numbers spreadsheet on iPad to preserve security? In apple discussions there is a suggestion to put spreadsheet in trash (supposedly at bottom). There is no trash at bottom. Deleting the spreadsheet doesn't close it, it deletes it.
In short - if going back to the linen background doesn't ask for the password again, you may have to force the issue in the same manner as killing a program that is crashing and needs a clean start.
Using multitasking, you can try to force the Numbers app to quit on iOS and see if that requires entering the password before viewing that spreadsheet.
This shows how iOS behavior of normally resuming exactly where you left off can have unintended consequences if the app should also have a check against a password and a time out period.
When you open such a spreadsheet you get informed that it will become a "Numbers for iOS" spreadsheet and you can Open a Copy, or just Open it. If you choose to Open it, you have converted it to Numbers for iOS, and the password is permanently removed. If you choose to Open a Copy, only the copy is converted and has its password removed. When you are done viewing it, delete the Copy, and you will still have the password protected version in iCloud.
Unfortunately, it means that if you want to preserve the password, you only have read-only access to that file.