What are "transporter locks" based on, though? If "life signs" means "body warmth", fire would be an effective mask, preventing transporter lock and thus immediate rescue.
Though Picard said Robert and Rene "burned to death in the fire", it seems more likely that they succumbed to smoke inhalation. Perhaps they were dead long before there was any alarm tripped. I recognize that modern detectors can trigger on heat or smoke, so why don't futuristic systems do at least that much?
What would a civilian home fire suppression system look like? A force field generated around the fire? Would the Picard family chateau have a modern system like that? Would it have the power supply, the generators, etc?
And how or why did Marie survive?