Not sure what I was thinking. I thought you were seeing them in Network and wanted to drag them into Locations.
Any file server that is advertising itself via Bonjour should show up in Locations.
It's possible to remove items from that list by dragging them out.
If you go to Finder Settings, Sidebar as directed before, the checkbox will not be solid. You have to de-select and then select to bring them back. If it is solid, then it isn't seeing those machines as file servers.
If they are not advertising file sharing services via Bonjour, they don't show up in Locations. I've seen servers drop in and out, so it may be a network issue, possibly the server sleeping.
There also seems to be an age-out from the list. They don't disappear immediately when it doesn't see them.