Poltergeist in the machine?

My computer is a Bermuda triangle

I'm not familiar with that manufacturer and model 😄

Do you have a taskbar? Did they disappear from there too?

I run KDE plasma desktop on my laptop, and my home and work monitors have different geometries.

On display attach/detach, the desktop moves windows around rather intelligently but sometimes the geometry messes up and the window may end up atbsome ridiculous coordinates outside display areas.

On a desktop computer with multi monitors, the displays are loaded one after another at boot or resume from display-off mode, so the same geometric bug can still trigger.

Do you use the "Restore Previous Session" feature of your web browser between reboots? Do you use keyboard controls to open new windows like Ctrl+N?

If yes to both, chances are that your mystery window had remained part of that continued session, and was reactivated with a Ctrl+Shift+N or equivalent. Shift is right next to Ctrl so if you fat-fingered a Shift at the same time that could explain the apparent necromancy.

As to where it went originally, you could have moved it to a different virtual desktop, moved it off-screen accidentally or, heck, even misclicked or miskeyed to have it actually close. A reboot would then have closed it for certain, except for the fact that it was still lurking in your session.

Another thing that can cause windows to move around is when a HDMI or DisplayPort monitor disconnects. Unlike with older connection types, those disconnecting is detectable and modern window managers try to "save" windows by throwing them on any remaining active monitors. If you have both types of monitor connection, or had at some point in the past, it could be that your mysterious window was lurking on a screen you couldn't see but the system still thought exists, at least until it was closed for certain.

Admittedly unlikely, but you could rule that out by checking the system monitor arrangements in the settings.

what DE/WM are you using? is it wayland based? I know sometimes if xwayland messes up it'll result in stuff like steam, discord, games, etc just showing a blank screen/see through to the desktop.

That's just your past coming back to haunt you.

If you use Firecat, they do that from time to time. You might want to put a location chip on them to know where they go. Careful they may bring some dead Firemice windows back on your screen.