up to no good
(idiomatic, litotes, understatement) Misbehaving, being mischievous.
He seems like an angel on the surface, but I can tell he's up to no good.
Has anybody ever seen a dramatic critic in the daytime? Of course not. They come out after dark, up to no good. P. G. Wodehouse
You have always been smart. You have always looked for the worst in people, and have been quick to notice when people are up to no good... But in the case of your children, you are blind. Federico García Lorca
Age is always advancing and I'm fairly sure it's up to no good. Jim Butcher
Computers, it seemed, could be programmed to do almost anything but sense when someone was up to no good. Daniel Abraham
An idle woman is up to no good. Corsican Proverb