There IS a check valve right before the turnoff valve before the hopper. It goes: pressure tank > ball valve > shutoff valve (green handle) > hopper > shutoff valve (green handle) > ...
You can't see it in the pictures, but it's there - it's about 8 inches of thicker greyish piping behind the carbon filter with a round center. That's what you mean, right? It's a little ball that keeps water from backflowing into the pressure tank? I have them in my aquarium filter stems.
Maybe the center tube was loose in the hopper, causing some of the mush to slowly leak out? It took me a half hour of wrenching and turning that thing to get it out when I cleaned it, maybe it was not in all the way.
I don't use a garden hose or have any outside watering adventures; I don't even have an outside faucet. If something was leaking I'd see it in the gallons remaining on the softener, wouldn't I? It never got below 400 gallons. And there is no moisture below the tanks except for the sweat on the tank bodies themselves; I'm sure I would see (or smell) a leak around the hopper or mixing tank.
I'll call later this week when my check comes in.