Discovered early this morning, when the timer turned on the light over Tinga's cage.
When the box in the kitchen is exposed to ordinary visible light, it emits muons.
Raising the threshold energy* and eliminating an intermediate quantum state** suffice for the moment to inhibit further emissions.
* Making the sides of the box taller.
** Taping over a discontinuity in the cardboard, which could be used as a step.