Red tags are used by many utility companies to identify a dangerous situation. A red tag is most commonly used for gas hazards - so at your stove, pool heater, or heating unit (if you have one that is gas - if you have a heat pump then you don't). If you are ever house shopping and find one of these, be very careful to research it before you ever purchase the home.
Whenever on a home inspection here in Orlando, one of the things I inspect are the exterior outlets. If exterior outlets are not properly installed, then they can be a real safety concern.
Exterior outlets need two forms of protection at the very least. First, they need self closing "doors" like the one seen in the picture to the left. They also need to be GFCI protected. If those two safety measures are not there, beware.
Mike McFadden, Certified Master Home Inspector in Orlando, FL