I agree about how much other stuff
someone could "borrow" from your campsite. I remember one place we stayed, the site next to ours had so much outside(bikes, toys, chairs, tables, fishing gear, boat,.....) I just knew somebody was inside that trailer. Never saw anybody the whole time we were there!
I too have never missed anything from my site either. But, I remember when I was a kid,
we would leave the house unlocked when we left for a short time. Now, we lock the door behind us everytime we come inside!
Good idea about the lock on the power box.
The campground could possibly question you about locking the lid closed. If you need to quickly shut the power off, remember where the key is!