Barometers?
There are barometers that are drop in replacements for most of the temperature sensors in a laptop or phone. The cpu that reads those isn't the main processor so it can run a very long time and is running in a semi-sleep mode most of the time when the laptop is "off".
Unless someone starts doing real world boom tests in simulated luggage compartments at typical flight pressures, I think I would prefer to have the device above the floor where someone might have the ability to do something rather than below it where only the fire suppression system can help out.