Yiddish: When the cat's asleep, the mice dance.

SOURCE: Kogos, 1001 Yiddish Proverbs.  Compare:

.אַז די קעץ רײַסן זיך פֿרייען זיך די מײַז
Az di kets raysn zikh freyen zikh di mayz.
When the cats quarrel, the mice are glad.

SOURCE: Stutchkoff, Der Oytser fun der Yidisher Shprakh

‏װען די קאַץ שלאָפֿט, טאַנצן די מײַז.

Ven di kats shloft, tantsn di mayz.

When the cat's asleep, the mice dance.