I had danios in with my goldfish and found that 2 out of the 6 dissappeared, found one dried up behind the tank months later, never found the other.
why did that happen
Danios are active fish, if you dont cover the tank properly any fish can jump out- most of the time its because they try to catch an insect flying above the water surface and accidently end up jumping out the tank, or because they got spooked by somthing and jumped etc.
Small fish take a very short period of time to decompose in tank water, many fish will also eat their dead tank mates even if they are not predatory fish, so its likely that the other danio died, for whatever reasons, and either rotted away or got eaten up- or its still lying around somewhere outside the tank.