I was reading about Cashaa, that was hacked and 336BTC were stolen. But the hacker can't move them right? He will leave footprints. So why should hackers steal them since they can be found? For example if they want to convert them to fiat.
The only way I think you could get by from this felony is to keep them on very long term. So long that it may be forgotten. But still, it may.
Hacking exchanges and accounts are not easy. Even they have knowledge in computer software or intercepting accounts, it may take time for them to steal some privacy such as email and passwords. In fact, even making phishing site is really difficult to do. That is why hacking is a one time big time event. Imagine, they took a month before they discovered the internal security of an exchange and they only want a one dollar of the customer. It is really useless and waste of time.