Spock, my first betta, loves getting into trouble. I had previously found him inside my Tetra Whisper 10i after being gone for 30 hours, but he was alive. Found him today in there AGAIN!
There's no easy way for him to have gotten in there. The outlet lip is above water level, and there is a baffle sponge rubber-banded across it. He would have had to jump into the filter purposefully. ARHG!
I've since moved him to McCoy's section of the 10 gallon, the middle, which does not have a filter. Hopefully he'll keep out of trouble there! I am certain that McCoy is not the kind to be leaping into turbulent waters for fun.