The theft lock is feature of the radio so that if there is a power loss and the theftlock is activated you have to enter a 4 digit code to "reopen" the radio. The theftlock sign will show up on the screen if it is activated. I THINK you have to push the two and the four buttons at the same time to enable you to enter the four digit code. Someone with one of these can help us out here and verify that piece of info. Four digit code like a PIN # or birthday, something like that.Drew