Hickory Dickory Dock

There's more than petty theft going on in a London youth hostel...
An outbreak of kleptomania at a student hostel was not normally the sort of crime that aroused Hercule Poirot's interest. But when he saw the list of stolen and vandalized items - including a stethoscope, some old flannel trousers, a box of chocolates, a slashed rucksack and a diamond ring found in a bowl of soup - he congratulated the warden, Mrs Hubbard, on a `unique and beautiful problem'.
The list made absolutely no sense at all. But, reasoned Poirot, if this was merely a petty thief at work, why was everyone at the hostel so frightened?