Usually they will cannibalize each other only when dead. Could the mouse have passed away and you didn't notice? Either that or like the other have stated, check the water bottle. They will also cannibalize each other to get re-hydrated.
Ball Pythons:
0.1.0 Bumble Bee- Schnapps
0.1.0 Pinstripe- Penny
0.1.0 Super Cinnamon- Cleo
0.1.0 Albino- Daiquiri
0.1.0 Low White Piebald- Hershey
1.0.0 Low White Piebald- Reese
1.0.0 Paradox Super Cinnamon- Dash
1.0.0 100% Het. Albino- Jester
1.0.0 50% Het. Albino Jake
One, is if you have two or more males together. They will kill and eat each other until there is only one left quite often.
Another, is extreme stress. How are your mice set-up?