Are we sure that an explosive decompression has an explosive sound? Having experienced bang chambers I don't recall particularly loud noises. Of course that could be a function of scale. It can also be a function of the low speed of sound at low pressures. Mind you, I have not experienced a bang from 8,000 feet.
Regarding passenger unaccompanied baggage. I submit that this is a regular event. Consider how often your luggage fails to arrive with you? It usually arrives a day or more later.
Now it is clearly impossible for a passenger to plan this but not impossible for someone to tamper with it while it is air side. Remember the nickname of Thiefrow years ago?