Unknown Windows SID Mapped to Proxy During SSIS Package Execution
Check the blog post I wrote concerning how to identify this S-1-9 SID.

From that post:


  This can happen as a result of using EXECUTE AS USER = 'SomeUser'; to switch context to another user that doesn’t have a server-level account. 


I imagine you'd see this exact scenario for contained users.

There is also an MSDN post from 2006 showing this SID.