Those under gravel filters were notorious for missing fish incidents! I had one years ago and always has smaller fish go missing, then reappear magically. I never did figure out how they got under there, but they did. I once found a kuhli loach swimming around in the lift tube. My best missing fish story happened when I was just a little kid. I had wild guppies and one day while cleaning their little 5 gallon tank, one big pregnant female went missing. I was frantic! My mom made me stop looking for her because we had to leave for a few hours. When I got home, I resumed my search and found her in the garbage next to the sink--she had landed perfectly on the plastic wrapper from a toilet paper package. Still alive, I put her back in the tank and she lived for another year!