Hi, 

We recently migrated to a new enviroment and found that there are still some clients reporting to the old SEPM which has since been decomissioned (no restore available.

What would be the best approch of finding and moving these list clients to the new SEPM?

Also is this statement true:

"I think in SEPM there is a way to audit against active directory to see which computer objects in AD OU’s have sep client. That might be a more comprehensive way to see if we have systems not connecting or updating, or systems without agents. "

Setup unmanaged detectors:

How to enable the Unmanaged Detector in Symantec Endpoint Protection Manager (SEPM)

Configuring a client to detect unmanaged devices

Keep in mind that you need one on every subnet.

Then you just need to replace the sylink with one from the new SEPM. You can do this from the SEPM

Restoring client-server communications with Communication Update Package Deployment

You can used unmanaged Detector find sep client where sep client not installed

What does it mean to set a client as an Unmanaged Detector?

https://support.symantec.com/en_US/article.TECH105722.html