Missing Baby Cory Presumed Dead

Sorry to read about this.

You've taken the right course of action in doing the ammonia test, doing a large water change, and checking under the decor. The only other suggestions I have, on the basis of current, recent and/or past experience, is to:
* check behind the filter, in the filter bracket (if there is one) and between the filter suction cups;
* check around the light fitting (although unlikely to cause an ammonia spike, it's possible a fish can catapult up and get stuck there);
* check the outer surrounds of the tank (again, although unlikely to cause an ammonia spike, it's possible he leapt out.

Shrimp, snails (not nerites, though, which are herbivores) and other fish could be responsible for eating him, unfortunately - any bones from such a tiny fish would be scarcely detectable.

Sorry to hear about your cory.

Thank's but it's bit hard on you' once you think you're out the woods at 5-6 months old.

Touch wood the other 3 are ok but 2 are a lot smaller than the other one' who is hard to tell from the other's.