As far as having it's on a "little" battery box this is true on utility trailers over a certain weight that are required by state laws to have brakes. This could be as simple as a small 12V lead acid battery in the 5 to 7 amp range. The battery sole purpose on the trailer is to power the Emergency break away. Travel trailers generally have a need for a battery so therefore the Breakaway is wired into the 12-volt system of the trailer.
As far as the person mentioning his missing Breakaway cable and pin, someone may have stolen it because he needed it, someone could have been doing it as an act of vandalism, it could have been teenagers pulling a prank. I heard a story once where someone who stopped at a rest stop said they saw teenagers hanging out around the front of their trailer as they returned from the restroom. They were looking at his trailer tongue. They saw him looking at them and they immediately scurried away. The person walked over and started looking at his hitch and found it had a pin missing from one of the spring bars on the weight distribution hitch. Most likely one of them had pulled it as a very dangerous prank.
When towing, anytime I ever stop for gas, food, groceries, use the rest stop, or leave the truck and trailer for any reason, I always check the hitch, all pins, trailer plug, chains, safety pins and break away cable upon returning.