My main computer has two screens and I like my panel on the second screen (the right one). Today I logged in on another machine, which has only one screen, but mounts my home directory via NFS. The panel, though running, was not visible at all -- presumably, because it wanted to appear on the screen #2, which did not exist. That seems silly...
For now this is intentional: https://git.xfce.org/xfce/xfce4-panel/tree/panel/panel-window.c#n2189 If you want your panels to "move around" with you, you have multiple options. 1. Use the "Automatic" option. Upside: this means your panel will show on the connected displays, even if one is missing. Downside: when you're back at your own setup the panel won't magically move itself to its old position again. 2. Use xfce4-panel-profiles to save your panel layout and restore it when you're home. 3. Use the "primary display" feature and just set your right display as primary. Then you have no downsides! For now those are the only options you have and to be honest I'm not sure I wanna get into changing this behavior (to some extent it was a mistake to even offer specific displays...) as it would imply a similar kind of work as I did for multi display layouts and remembering them (which was a long and painful road).