Battery-operated Natural Gas Detector?

[Joseph Taub]

We rent rooms in our house, and one morning woke up smelling gas *throughout* the house. Our tenants somehow had not turned off the gas range all the way after cooking. These tenants also liked candles....

OK, so I guess no more candles allowed. Lesson learned. But even so, natural gas could be ignited by other things too.

It seems like every natural gas detector I see online is "plug-in" only. Well, that's not really tenant proof if the tenant can just go "Oh, I need this outlet for a few minutes to charge my iphone so I can post to Instagram," then totally forget about it. Then cook. Then blow up the house.

I would just replace it with an electric range, except we live here too and use it to cook, and we are in CA where electricity costs more than bitcoin. 

Anyone have any ideas for "tenant-proof" natural gas detection and alarm?