Every security company has them -- those yard signs and window stickers you display to let trespassers know that your home is monitored by a security system . But starting Tuesday,  ADT is offering  a  security device  I've never seen before: a smart yard sign that lights up at night.  Powered by AAA batteries, the Live Light connects to ADT's hub and can be controlled via the ADT Plus app. You can turn it on at night, which helps notify potential burglars who would otherwise miss a sign in the dark. And if the security system alarm is tripped, the sign will automatically turn on.  A representative from ADT didn't immediately respond to a request for comment.  The Live Light costs $50 when bundled with a professional installation, and right now, it looks like that's the only way to purchase the smart sign.…