You Can't See Them
We track the case we are after and find that it has been taken by elves, who want the information inside it for themselves. With a bit of diplomacy and compromise, we strike a deal to at least be able to see what is inside the case before they whisk it away to their lands.
The case has already been taken to an elven enclave within the city, which we are led to. The whole of the enclave looks like walls to us, but the elf leading us walks up to a certain spot, presses something that we couldn't see, and a door opens.
'Pah, elven secret doors', spits Thrak. 'Dwarf secret doors are better.'
'But they're generally just stone doorways, aren't they? Failing to put a door in the hole doesn't make it secret.'