When a client is deleted or purged due to inactivity from the SESC console, when they are added back, they are not placed back in the Group they were previously assigned.
When creating the install package to be used for installing the SESC agent on clients, a Group is selected to put the new clients into by default. This Group is where clients will return when re-added to the console.
The systems will end back up in the Group assigned in the installer used to install the client. If that group has since been deleted, it will be placed in the root Default group.