Hey DMF
When I purchased my bike, the previous owner gave me two sets of keys, BOTH that look like this.
(http://i.imgur.com/aPHHyAh.jpg)
Both work to start the bike, but there's also a small metal key on the attached keyring.
Is this key important at all to the function or maintenance of the bike?
To me, it looks like a regular lock key.
Maybe ask the previous owner? It isn't a Ducati part.
That small key looks like a pad lock or something similar. It's not for the bike, in stock trim at least. The big Ducati key should operate the ignition, open the filler cap and open the seat latch-lock.
My only suggestion is the test the Duc key in all 3 before losing track of the small one. It's possible that the seat lock might have been replaced or something like that...