I've directly encountered the gas-hob-on-but-unlit scenario myself with someone whose sense of smell is virtually completely gone, and it terrifies me.
(How far gone? They are unable to distinguish, or detect, the sents of various Dr. Bronners soaps, directly before them.)
And mercaptan[1] (the oderising component in municipal natural gas) is among the most detectable scents in existence. The reasons why it was introduced[2] and its alternate uses[3] are worthwhile reads. As I've noted elsewhere, virtually all safety regulations are written in blood, this one the 295+ souls claimed at New London.
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Notes:
1. https://en.wikipedia.org/wiki/Methanethiol
2. https://en.wikipedia.org/wiki/New_London_School_explosion