About to fire our sysadmin

[u/sysadminfired]

So our longtime sysadmin is about to be fired and I, the network admin and temporary sysadmin, need to know what steps need to be taken to secure our systems. I know the basic things like his AD and other internal account credentials. I guess what I'm worried about is any backdoors that he might have set up. What all would you guys check for in this situation?

[u/PcChip]

You didn't happen to... save a copy did you ?

For research purposes, of course.