I had configs and layout documentation stored on a hidden server on the network. I backed everything up to it. Then I archived it and encrypted it so it could be stored in plain site on several servers, on tape backup, and store the tape in a bank vault. The network didn't change much, so this was a viable option. But when the network went down, the other admins had no clue how stuff was configured because they weren't doing the backups like they were supposed to. Some managers were let go. I figured out what the problem was and reloaded the router with the config off my hidden server. When I left, I still don't think they found the server.
The backup server is a Raspberry Pi v1.0. It's in a small, non-descript case with a label that has the IP address and a message that reads "Critical network monitoring equipment. Do not disconnect."