Far out, very lucky. The amount of stress that sat phone has saved you and them its worth its weight in gold (although they nearly are anyway)
About 12 months back I came home from work and the house stunk like gas, and all was quite. Rushed around and opened the doors and windows closest to me then went to the stove top and found the gas on the smallest burner was on. Went into the bedroom and found the misses, the 2 year old and 3 month old were asleep in the bed (that was a scary sight until they woke up). I told her the stove was left on (they had been in bed for about 2 hours) and it stunk like gas and to get out side. On the way out I said cant you smell that? She couldn't smell a thing.
So it seems if you asleep or the leak is very minor you will not notice it building up.
I pulled that 6month old stove top out and put in a auto shut off style and always turn off the gas cylinder on the van before going to bed.
So you cant rely on you nose to save you, really a gas detector is the only way to go.
Hope all goes well for your parents Holden_man, will be a long trip back for them.