I know so many absent-minded musicians who have left instruments in cabs and on subways. All of them so far have been lucky and either found them where they left them or had them retirned.

If you find an instrument people, remember it could be someone's livelihood!

And they usually can' t sold. Especially something like a Stradivarius. They are known all over the world and are registered. It would be like stealing the Mona Lisa and then trying to sell it.