Do you have any decorations with holes in the bottoms, like so they will sink, sometimes fish can get up inside of them, even when they seem small?
Even heard of them getting inside and getting big so they didn't fit out the hole anymore.
Had a Cory go missing and thought was dead, in my daughters tank. We had taken out the other fish, couldn't find the other cory. We looked again and she looked again and still nothing. I am a good hunter.
She unplugged the tank after a week and another check. No heat no air no food!
It was around 3-4 week later she hadn't cleaned out the water yet. I noticed the tank and asked her if I could use it for some guppy fry.
Then when I went to get the plugs out from behind the cabinet I saw something zip through the dark water!!!!!!!
There was the little guy, a bit thin but very happy to see humans!!
Still going strong, a year and a half later, grew twice as big even and reunited with his friend Cory.
I had a black Kuhli stuck in a log type thing for maybe more than a month/anyway he was missing for a month
Thought he was dead, he was getting big in there!
So be super careful and quadruple check. There will be a skeleton if it died. They are very bony.
So it could be hiding really well.