I have a tight, weighted lid on my tank but I guess there was an opening JUST big enough for one of my senegals to hop out. Found him/her on the floor this morning as crisp as can be. Is there an actual reason that they do this?
As an inexperienced owner I would think that something spooked it. Maybe another fish touching/biting it at the dark ? I know from my experience that my fish and especially my Albino Senegals dart around like crazy when touched. Another reason would be bad water quality but doubt that was the case with your fish. I also seen them seemingly "play" by using the filter current as a treadmill and swimming upwards to it and breaching the surface in the process.
I just had the 3 senegals, two regular and one albino, in a growout before they go in with the gars in my 75 and I thought bichirs liked to huddle together nd ive seen no aggression between them except at feeding time. These guys are in my office right now so ive had plenty of time to watch them.
It's very important to make sure there is no openings for Bichir to jump through because it's not always that their spooked. They like to escape. I've personally had Polypterus jump out of aquariums did save one before she dried out.