Red Flag #1: The banks leasing the Silver will become concerned about hard inventory levels and the ability for repayment at some stage (its already too late however).
Red Flag #2: In order to not create a run on Silver, they will gently increase the Silver lease rates over time as to not scare other banks into the realisation of the same problem they are witnessing. It's important they do this relatively slowly... the last thing these lenders want is a run on Silver as it diminishes their chances of getting the physical back into the vault.
Red Flag #3: As the lease rates increase, it doesn't make sense to keep the monies earned from selling the Silver in the first place in the open market and lease contracts will not be rolled over.
Red Flag #4: Metals will be bought back on market and delivered to the leasing banks.
Red Flag #5: As this starts to happen en masse, Comex and other Silver exchanges will default on delivery and at that time the cat is out of the bag and the Silver rush will ensue.