Did you use strong and unique password for your coinbase account and associated email account? (Unique means: never ever used for another account!)

Are you sure the computers from which you had access to coinbase are not compromised with some malware/keyloggers/whatever malicous?

Did you ever have access to your coinbase account in an unsecure network, e.g. public (W)LAN?

Did you access your coinbase account with your smartphone? Do you know all your apps on your smartphone?

Disclaimer:
I don't know coinbase, so still I can't see who is to blame here or how you actually lost your bitcoins.


How do you know who is the thief, provide more details on this!