Maybe Nevill was tired. Hadn't been out doing some more work on the farm? Maybe he didn't see the gun, or maybe he thought he'd put it away in the morning.
Or he was too tired to notice that his son had left a loaded gun on the settle, or the gun wasn't immediately visible, or he saw it and thought, quite reasonably, that Jeremy, who'd been the last to use it, would put it away, then became distracted and didn't check.................all of which is only possible if the gun was left out.