Oven or stove pilot light, timer for lights (the owner was out of town), almost any relay not sealed or set up for explosive atmospheres, like the fridge compressor kicking in. If the mixture was just right, anything which might produce a spark.
I was thinking thru my home. It was built 5 years ago; about the same as the neighborhood where this one blew. Everything is all digital, including electronic pilot lights. Fridge compressors are sealed units.
Thinking more in terms of safety and getting rid of ignition sources. An old mechanical light timer is a good suspect. We do get our gas appliances serviced every 6 months.