I do shut them when I leave the boat, but sometimes I forget. Other times I start driving home and can't remember whether I shut them or not. Must be getting old.

I don't know how it happens, but the thru-hulls can also fail. The previous owner of my boat told me that one of the seacocks had failed while underway and nearly sank the boat. The Coast Guard responded and gave him a bunch of oakum to shove in there (no plugs?). He replaced both seacocks, but maybe his wife didn't like the boat too much after that.

Still have the oakum.