I was able to determine what was going on: Windows 10 does not have a c:\windows\panther directory by default but it does get created later after the OS is deployed and the system boots the first time. Panther was also was created in c:\windows\system32\sysprep.
So when building a deployment template to inject your unattend.xml file into the proper place so that Windows 10 processes it, the proper directory is c:\windows\system32\sysprep. (different from windows 7 where it is in c:\windows\panther)
When windows runs through its first-boot phases it will copy the unattend.xml file only to C:\windows\panther and leave the one in the sysprep directory that you injected. So if you want to remove it, you have to delete it from both places.
I also ran into an unrelated issue where the CTOS step was failing with "not able to determine sysprep drive". This will happen if you either a) did not use a BCDboot step to set your up your boot partition and make it active. b) no pause /action between the BCDBoot and CTOS. The CTOS steps happens so fast it tries to start before the partition is ready. A 5 sec wait fixes that.