I live alone so nobody used my computer.  So, somebody I know using my computer is not a possibility.

There was no coinbase login record other than the logins I initiated.  Also, someone had to access my gmail in order to approve login every time. And there is no google account login history other than me logging in. 

So, the only thing I can think of is malware.  I had AVG free version that scanned computer but found no malware.  So, someone must have been able to hijack my computer remotely through a sophisticated malware that was undetected. 

What I don't understand is this. If would someone go through all that work and not try to make any money?  I have Paypal accounts with cash balance, multple bank accounts.