Do you have "Don't lock system" option (S2U2 Settings > Advanced) checked? According to A_C's FAQ of "What is "
Don't lock system", some function can be activated even when the device it's locked. Here is the original FAQ from
http://67.43.3.117/~acs2co/S2U2-faq.html:
"A. Since v2.20, when S2U2 locks the system, the system will behave as it's locked by the system built-in lock. But on some devices, it may lead to some not-welcome behaviour when locked: for eg, the backlight will be dimmed; on some devices, unlock will always go to the home screen. If you want the device behaves as v2.20 or before, turn on "Don't lock system" But there is a draw back when it's on, some keys or system functions can be activated even it's locked."