I wouldn’t worry about it too much. My green anaconda, mainly when I first got him, but he still does it, just not as much, yawned frequently. I worried about it but no sign of mucus and still feeding so I just let it go. Same thing as you, I have cameras in my cages. That’s when I would see it. I’ve seen my daughters ball yawn a couple times randomly as well. Numerous reasons, just make sure they aren’t holding their mouth open and your not seeing any mucus.
wait until you hear/see them fart. That’s freaky.