Q:

Where did my danio fish mysteriously disappear to?

I have 8 white skirt tetras, 8 albino cory cats, and had 8 danios. Now I have 7 danios. One mysteriously disappeared. I am very doubtful it is hiding because my danios don't do that; all they do is swim at the surface. 
It is impossible for the fish to have eaten the danio. They are too small.
It is impossible for the fish to have jumped out. The tank is fully covered. I checked around the tank, searching for a good 30 minutes - nothing found.
My fish isn't dead. The cory cats would have easily started feasting on it. They eat anything that's at the bottom, essentially. (Perhaps it died while I wasn't around?)
My filter doesn't contain the fish.
No sign of a fish skeleton ANYWHERE.
What could have happened to it?

A:

It is actually not uncommon for fish to disappear, seemingly without a trace.  It has happened to me a few times.  Sometimes you eventually find out what happened, sometimes you don't.  There are several things that could have happened to it.  Each of these things has happened to my fish at some point in the past.

It might be hiding.  Some fish are really good at this.  We had a kuhli loach that was very good at hiding in the gravel.  One time we didn't see him for a week, and we thought sure he was gone, and then one day he was swimming around the tank again.
It could have jumped out.  Even if you have a cover on, if there is a small gap around the filter or heater cord, a fish can jump through it.  We had a glass bloodfin tetra that disappeared, and we eventually found it on the floor underneath a book rack next to the tank.
When a fish dies, often other fish will eat it very quickly.  This can happen fast (a matter of hours).  It doesn't mean that you have aggressive fish, it just means that once your fish dies, the others see it as food.  They are fish, that's what they do.  This is probably the most likely scenario in your situation.
On the other hand, the fish might be the victim of an aggressive fish.  This is always a possibility, even if the fish are supposed to be peaceful, and there isn't a big size mismatch.  We had a molly that, for whatever reason, always attacked any other molly in the same tank.

