“ An ambitious spy can delete the preference file in “~/Library/Preferences/ByHost” that triggers the password prompt (com.apple.screensaver.*.plist) so a computer left later unguarded can be accessed. This is a known design flaw and was supposed to be remedied in 10.5 but unfortunately it wasn’t.
macosxhints.com - 10.4: Possible security issue with wake from sleep
macosxhints.com - 10.4: Possible security issue with wake from sleep