From the German, schtoloßten. This is a combination of the words lost and stolen. It refers to instances when:
1. You lose something and it is stolen (usually shortly) thereafter.
2. When you lose track of something and are not sure if you lost it or if it was stolen.
If there is a chance of it turning up then it is not stolosten, it is just lost. For something to be stolosten it has to be assumed in the possession of someone who is not the owner and who has no intention of giving it back.
This is usually used in terms of cell phones, laptops, purses, wallets, and other valuables that are quickly stolen if they are left unattended.
My iPhone fell out in the cab, I called the company but they have no record of it being found. My iPhone is stolosten!