Could be as simple as someone cleaning the stove and accidentally turning on the burner(happens all time when cleaning my kitchen gas stove) since the gas was turned off at the tank no one would have known without seeing the burner knob,until the tank valve was turned back to the on position. I normally always leave my BBQ grill tank valve in the on position. Last weekend I put a new/full propane tank in and grilled out once..all burners were in the off position/tank valve left on(like I've done for years)...came out a couple days later to preheat the grill... zero propane left in the new tank. Apparently my grills knobs are no longer fully shutting off....no biggie.. I'll simply turn off the gas at the tank from here forward.