I've caught myself doing this a couple times before and a question at AUGI reminded me of it. We can temporarily hide things using the Temporary Hide/Isolate feature. When not in use the icon just looks like glasses, more like sunglasses when it is in use, plus the turquoise blue frame around the drawing area and the slight shading behind the glasses too. One of the options available to us is listed at the top, Apply Hide/Isolate to View.
I've clicked on that when I really intended to click the Reset Temporary Hide/Isolate option instead. Later on I end up wondering where "that thing" went. Reveal Hidden Elements (little light bulb) helped me out.
If whatever you were missing shows up with this mode active then you did the same thing. The Apply Hide/Isolate to View option either applies your temporary condition to the element selected or the category depending on how you temporarily hid it to begin with. The end result is the same as if you used the right-click option Hide in View and either chose Elements or Category.
Next time "that thing" goes missing the little light bulb might need to go on.