Most fish when added to the tank will hide, they can really hide in a piece of driftwood if they want to. Even in my qt tank with just one piece of wood, they could still hide.
They are well known jumpers, if you have a dog that is into eating things from the floor then yes that's also a possibility.
And c) could be it too.
That being said I got three and lost two before the first week, and they didn't float to the top.