Best of my knowledge and experience the MEL security goes thru great lengths to check and confiscate liquid, lighters and matches.
I think there could be some weight age to post #48. Seems to be some time delay inflammable device dropped into the bin. What is scary is that this device seems to have made it thru at least 3 screenings of which 2 were specifically looking for it.
I seriously doubt it is a smoker as the alarm would have gone off a few puffs after he lit up and before he exited the lavatory.
Wonder if some forensic analysis was done on the bins to see what was the source of the ignition?