[be] up to something

to be involved in a particular activity or to be planning something, often with a sense of secrecy or suspicion

I think he's up to something because he's been acting very secretive lately.
Why are the children whispering in the corner?They must be up to something mischievous.
The sudden flurry of activity in the office suggests that the team is up to something big.
She gave a sly grin, hinting that she was up to something exciting for the weekend.
The sudden burst of creativity in her artwork signaled that she was up to something extraordinary in her studio.