How do I close a password-protected numbers spreadsheet on iPad to preserve security?
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.


Close out any spreadsheets you have open so you are at the main screen in Numbers with the linen background.
Double tap the home button to bring up the multitasking bar
Swipe left or right (if needed) until you see the numbers icon
Tap and hold the numbers icon until they all shake and have a small red circle icon with a minus sign in the left corner of the icons
Tap the minus icon on numbers to cause iOS to terminate numbers and force it to launch from a clean start the next time (rather than resuming the prior state)


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.