Slightly worrying – disappearing Word doc

It's likely that there's no way to get you an answer here, unfortunately. Not without far more access auditing enabled than OS X likely even supports.

As for another approach for finding the file (or trying), try Command-Space or the Spotlight button, and enter the filename into the search box.

Etrecheck can sometimes spot disk errors starting or other issues with the environment, but it's unlikely that disk errors will produce an isolated file deletion — usually the mess starts out with file corruptions and beachballs (the "spinning wait cursor") and with system performance degradation, and disk hardware problems usually get larger fairly quickly.

In general, your backups are your path out, because all hardware eventually fails. Including your backup hardware. Even backup hardware can get corrupted and become problematic to access and restore, and that backup hardware can and will eventually fail.

Usual response for this eventuality of failure: have backups. You have backups, which is goodness. For this case, can you fetch the file from your (presumably) Time Machine backup, or whatever else you're using? That'll also test that your backups are viable and functioning, which is also goodness.

FWIW, if you're running a manual backup, I'd switch to Time Machine or some other automatic backup, too. Manually-scheduled backups get skipped. I've skipped them myself.

As for this case, maybe a problem or an error with the Microsoft tools or maybe a file deletion request, or maybe the file save got retargeted somewhere else? I've occasionally deleted a file by accident, while clicking on several files in a group, or saved a file somewhere strange, it's not at all hard to do that without realizing it.

See if you can find the file using this program. If you go Finder/View/Show Path Bar, it will show you where it is located.

EasyFind – Spotlight Replacement

You are welcome.