Yea, all the sign does, for someone motivated enough, is tell them how much time they have. "Okay, they have a security system. Break in, grab the tv, grab the other stuff that's visible and worth something, then run"

At the end of the day... it will deter those that aren't interested in dealing with it. And for those that aren't deterred it just gives them the info they need to know it's a smash, grab and run situation.