I hear the possibility of backflow. The endless leak makes me skeptical though. I don’t think flume measure direction and I know the water meter won’t spin backwards but I would hope it wouldn’t register backflow as “consumed” water for billing purposes.
After the zones shut off, have you inspected the PVB for water coming from the top section? The way they work is a diaphragm shuts of flow and sometimes they can bleed down some water and can leak some water out in the process. Also, if the diagram section is failing, they can continuously leak. It’s possible that there is a difference in flow between front and back that could cause the PVB to react differently and only “fail” with the backyard.
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I second what @tmcgahey says, although I’d assume you’d see that leak… worth checking, obviously
Go to the lowest sprinkler in each zone. Look for dampness not explained by just normal drainage out the low sprinkler. As another commenter said it’s not that difficult to pop open diaphragm valves. I’ve had several weeping valves and I always find a piece of crud preventing valve closure.
If your system is older it might be a good excuse to just go through each valve.
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Replace the master valve diaphragm too. Those ports can get clogged or the rubber swell depending on water elements. It’s best to use a new one, but if you don’t have a new diaphragm and the old one is not torn, clean it up, use an eighteen gauge wire and push it through the ports to clear. Try your test again.
Are the Hunter valves the same age as the master valve? What model Hunter valves? It’s possible those need some attention too.
You may just have some valves needing attention all at the same time.
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