I managed to somehow turn off the visibility of slices. Don’t ask me how, but I did.
The slices still exist on the page (I can find them in the layer panel and I can select them by dragging a select box over their edge) but I don’t see the dashed outline.
It’s not file based, because my co-workers do see the slices.
Have you tried using the search menu?
I hadn’t.
That’s a bit of a hidden feature, isn’t it? 
It’s also in the View menu 4th item
Show Outlines? That didn’t work, for some reason. (I did try that… 
)
This same issue happened to me today! When I search “slices” I don’t see the “Show slices” option
Seems like this was some kind of experimental feature which is now removed. You can find plugins that do it.
Update: turns out it was a bug, the option is back.
I have the same issue and can’t fix it for the life of me. Is the search menu the menu in “Help” in the main navigation? Because I also don’t get the slice option there.
Thanks! It works now.