The Grauniad had it thus:
She said she was told the item had been swabbed and no trace of explosives found, so she “popped it into” her jacket pocket so “it wouldn’t go missing”.
Jeffery, who has worked at the airport for 19 years, said she later put it through an X-ray machine but could not see a detonator and did not believe it was viable.
The device was later passed to counter-terrorism police and, when examined by experts, found to be a “potentially viable” bomb containing nitroglycerin and nitrocellulose.
The question here is what "potentially viable" means in this context. Usually devices are viable or not, why qualify this with a "potentially"?