I am looking for data about stolen bitcoins. Is somewhere statistics about how many bitcoins were stolen etc.? I only know that bitcoins were stolen in Mt.Gox and Flexicoin...
There is no evidence that Mtgox has been hacked, it may be a fraud. There is only one bitcoin theft that is fully documented. You can find details here
For a bitcoin to move from one address to another, you require access to the private key associated with the address on which the bitcoins are "stored".
Stolen in this context means that the thief somehow had access to the private keys, and unless they were given to the thief, the device must have been compromised in order for the theft to have occured.
In other words, if the computer contains a virus or some other malicious code, you are at risk of having your bitcoins stolen.
As most people do not know if they have a compromised device or not, and the fact that the public address and private keys hold no personal information it is impossible to tell if the movement of the coins was a real transaction, or a transaction caused because the machine was infected.
Therefore there is only anecdotal evidence that bitcoins are stolen, and no real evidence.