in SFs dry season, my friend just leaves his windows open because his car got broken into so many times. Since hes left the window open he doesn't think his car got touched.
Leaving the sign leads people who might be considering theft to believe that (1) there is something inside that you are trying to protect, and (2) the car is locked and wasting time trying to open the locked door will set off an alarm and reduce the chance of them getting the thing you are trying to protect before being noticed. So, yes, of course it has that effect.